Strange, the site is supposed to make it impossible to double post in less than 24 hours. Saved me from a number of accidental duplicate messages, actually.
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The game came out in 2009 in China and in 2006 everywhere else. Trainee's Chinese so it makes sense.
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I believe all slots have associated files, but many share the same files. Old versions of NSMBe (5.0?) were able to change the file associations, which is a feature that was re-introduced in NSMBe 5.3. They can be changed to use unused files or added files so all background slots are unique.

Added files are far easier than they seem. With Every File Explorer and other tools, it's about as easy to add a file to a DS ROM as it is to add a file to a ZIP archive.

I believe quite a few hacks make use them, actually. Like the aforementioned Newer DS, Super Mario Endless Earth, Sarasaland Advance,  pinet's More Super Mario Bras. hacks and probably a few more.

They also cause no issues patch-wise unless you're using NMP patches (which would make added files pretty pointless in the first place, since NMP doesn't support code patches which are required to make use of added files). Unlike NMP, xdelta is universal: it can create patches that can turn any file into any other file, so it handles things like added files effortlessly.

By the way, if you're still short on background slots, asm Each Area has its own graphics will give you more than you'll ever need. Be warned though: it's a pain to use.
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There's an actor for that. You should be able to assign it to some unused sprite with the classid tool.
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Played some of this on a road trip. Got up to World 3-B getting every star coin besides the one in 2-5. Made a list of the issues I encountered on my phone as I was playing, hoping it'll be helpful. Kept it strictly to bugs and glitches, no design nitpicks, though I would've liked to include a few.
[List Moved to Lower Post]
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Game finished.
General:
*Physical checkpoints don't make a bottom screen flag appear until you change areas, at which point it appears at the level start rather than the checkpoint position. This affects every level with more than one area. Lazy fix would be to erase the bottom screen checkpoint graphic. That way, it wouldn't appear at the start point, but it also wouldn't appear at all. Harder but more proper fix would be fixing the code or using the Newer DS checkpoint code instead.
*The character selection arrows in the level preview screen don't go away after you're done selecting.
*The world map castle model has black polygons that cause z-fighting with the floor.
*For some reason, I'm unable to collect a Fire Flower as Mini Mario. I just go right through.
*Ground tiles in the desert, underground, sewer, snow, snowy castle, jungle, sky and underground lava tilesets (and probably more) lack the slope edge tile behavior. Players and actors will be stopped when going up slopes connected to ground. This also applies to the ground/wall edges in the sky tileset (and probably more). Come to think of it, looks like that desert tileset is mine. Oops. Will have to update it.
*Slopes in the desert tileset can't be slid on. Once again, my fault. Oops.
*The Mega Mushroom block uses the wrong graphic for a brief period after being hit.
*Shy Guys are glitchy as a result of being simply remodelled Splunkins. After being hit once, they will turn solid red. Use  skawo's Splunkin Remodel Code to fix this. This will allow Shy Guys to simply die after one hit.
*The door opening animation from exiting a ghost house is glitchy: it uses the NSMB door graphic while opening and goes back to the NSMBW one after.
*The airship model is too high-poly. As a result, only about half of it renders on a real console and a large amount of lag is produced. This lag breaks the loop for the tree animation.
1-2:
*It's easy to cause lag by killing giant Piranha Plants here.
1-Castle:
*The vine block briefly uses the wrong palette after being hit.
*Going back down the vine in the bonus area kills you.
*The door made my game crash (with crash handler) the first time I went through it. I was not able to reproduce the issue. I unfortunately did not take a picture of the crash handler, since I thought I would be able to reproduce the issue.
2-2:
*The starting entrance has a camera issue. The camera instantly shifts vertically once the pipe is exited.
*The bouncy bricks (pumps) don't show up when entering an area until you're out of the pipe. The first place in the level where it happens is the starting point, so using a regular entrance should fix it. The second place in the level where it happens uses the same tileset and backgrounds as the previous area, so the two areas could be merged together, which would fix the issue.
*The cork pipe looks darker than the pipe tiles it's connected to after a few pumps. Its bottom tile also overlaps with pipe tiles, which causes z-fighting. Not having it connected to pipe tiles would fix it.
2-4:
*The secret exit's castle doesn't have a 3D wall, so Mario never enters the castle and just awkwardly stands in front of it until fadeout.
2-5:
*Camera glitch on level completion.
2-6:
*The checkpoint entrance has a camera issue similar to the 2-2 entrance.
*Entering horizontal pipes from a vertically moving platform (here a pink mushroom) often causes Mario to enter off-center and stick out from the top or bottom.  MeroMero's Pipe fix for ASM-Template should fix that.
2-Castle:
*It's easy to cause lag by killing giant Piranha Plants here.
3-1:
*The start entrance has a camera issue similar to the 2-2 entrance.
3-Tower:
*The pipe bubbles die when defeating Bowser Jr. This can be fixed using  MeroMero's Choose which Actors to spare on level completion hack.
3-B:
*The start and checkpoint entrances have a camera issue similar to the 2-2 entrance.
3-B:
*The start entrance has a camera issue similar to the 2-2 entrance.
3-C:
*The start and checkpoint entrances have a camera issue similar to the 2-2 entrance.
*You can go back down the pipe in the sky bonus room. The entrance leads to itself.
3-Ghost House:
*The start entrance has a camera issue similar to the 2-2 entrance.
*The red flying block doesn't appear on level start.
World 4:
*There's a tree inside the last Toad House and to a lesser extent the cannon.
*The south-eastmost tree is floating in mid-air.
4-3:
*Start entrance has a camera issue similar to the 2-2 entrance.
4-A:
*The falling snow right after the red pipe with the block outlines will disappear until it touches the ground, at which point it will very quickly despawn. The player can still get hit by the snow. If this happens, the player will get stuck as normal, but inside invisible snow.
4-Ghost House:
*The red flying block doesn't appear on level start.
*For some reason, the ? Switch in the third room sometimes shows up as already pressed (though with full collision), with the Tilegod tiles being permanently present. When this happens, blue coins in the second room appear around the third door. It seems to be consistently reproduceable by hitting the ten coin block in the second room. My theory is that hitting the block is meant to make the blue coins spawn, but you accidentally gave the ? Switch the same event ID as you did with the blue coin event.
*Your usage of the slope edge tile behavior on the solid-on-top stairs causes upwarps when jumping from below.
*There is a gap in collision between the stairs and the ground im the first Star Coin room that it is easy to fall into and die.
4-5:
*Wall jumping up the one tile gap in the ceiling at the end of the second area for long enough allows the player to get above the ceiling and get softlocked as a result. To fix this, you can prevent off-screen wall jumping by giving your off-screen tiles an icy tile behavior.
4-Castle:
*The player can get softlocked above ceilings by wall jumping high enough. To fix this, you can prevent off-screen wall jumping by giving your off screen tiles an icy tile behavior.
*The soundset is missing Bob-Ombs.
*The Mega Goomba stands one tile above the ground before becoming giant.
World 5:
*There are trees seemingly floating in mid-air.
*There are trees going through the airship.
5-1:
*The player can get above ceilings by walljumping high enough.
*You can go back up the connected pipe before the checkpoint. It'll glitch out and release Mario inside the wall.
*It's easy to cause lag by killing giant Piranha Plants here.
*When using a shell to get the third Star Coin, the red blocks that are spawned will send the shell back the direction it came from, potentially hurting the player.
5-A:
*The start entrance has a camera issue similar to the 2-2 entrance.
5-Tower:
*Mario can be moved by stone walls on the checkpoint area above them.
5-3:
*The big Wigglers case lag when more than one is on screen.
*Ground pounding the block containing the ? Switch will make it fall on the giant Wiggler, but it won't move alongside it.
5-B:
*Pushing the pipe right after the checkpoint with Mega Mario prevents its entry. A second, identical entrance should be placed one tile to the right to fix this.
*The map Hammer Bro doesn't appear.
5-Ghost House:
*The map Hammer Bro doesn't appear on level start.
5-C:
*The soundset is missing Dry Bones.
5-Castle:
*The end of level cutscene is glitchy. It shows the World 6/7 paths unlike the World 2 cutscene and Mario goes down the World 7 path even when non-Mini (though he ends up in World 6 anyway).
6-1:
*The start entrance has a camera issue similar to the 2-2 entrance.
*The Hammer Bro can jump into the floor. Refer to this for proper Hammer Bro usage. The only situation where a Hammer Bro won't jump down is when there are exactly two tiles between him and the bottom of the "chunk" (set of 16x16 tiles) he's in.
6-A:
*You can go back into the entrance pipe. It leads to itself.
*The checkpoint entrance has a camera issue similar to the 2-2 entrance.
6-2:
*The start entrance has a camera issue similar to the 2-2 entrance.
6-Tower:
*The soundset is missing Bob-Ombs.
6-3:
*The soundset is missing Bullet Bills.
6-Castle:
*The soundset is missing metal fences.
7-1:
*Too many Piranha Plants cause lag.
7-Tower:
*The soundset is missing spiked balls.
7-4:
*The pipe to the secret exit can be broken with Mega Mario. This causes a soft lock, as horizontal camera scroll is disabled in that portion of the stage.
*The camera flips out if you jump to the right from the second Star Coin "platform".
*The soundset is missing fire Piranha Plants.
7-5:
*The soundset is missing Lakitu.
8-1:
*The start and checkpoint entrances have a camera issue similar to the 2-2 entrance.
*The soundset is missing Ka-Bombs, falling rocks and Podoboos.
8-2:
*The soundset is missing falling rocks and Podoboos.
*The bonus room exit has a camera issue similar to the 2-2 entrance.
*Both the start and the checkpoint red flying blocks spawn when starting from the start.
8-3:
*The soundset is missing Dry Bones, falling rocks and Podoboos.
8-4:
*The soundset is missing spiked balls.
8-7:
*The soundset is missing falling rocks.
8-8:
*The soundset is missing falling rocks and Podoboos.
8-Tower 2:
*The soundset is missing Bob-Ombs.
8-Final Castle:
*Black pixels appear in the bottom background shortly after level start (on top of the ones that are already there).
*The second area sets the progress path back to the start.
*The SFX for the last door doesn't play.
*Going through the final pipe crashed the game my first time through.
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Use the Find feature. Make sure you have it set to search for hex data. Also make sure your overlay is decompressed.
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Change the value at 0x2010 in Overlay 8 from 0x04 to 0x03.
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If you want 6>7, you'll also have to change the value at 0x2030 from 0x07 to 0x06.
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Seems to require a North American ROM that has been opened with NSMBe. A clean North American ROM won't work.
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Set up the asm template, put your replaces.x in your template folder and the worldX.c files Goombatlas exports in your source folder. Then, using the modified NSMBe, run make and insert.
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Likely a false positive, the file overlap detection is broken. Just ignore it.
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Yeah, you can convert the NSBMDs to DAEs with Apicula.
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Red snow might be a DeSmuME bug. I think there was an issue like that in Newer DS too, which should be fixable by tweaking 3D rendering settings.
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An undead hack sounds pretty scary.
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