somedit is an awesome resource for looking at game data, but it's incomplete and can't be trusted to make a functional ROM. Even if it wasn't incomplete, it moves stuff around so much that it wouldn't produce output that'd be compatible with any other SoM ROM hacks. While it uses real SoM data as input, it does not interpret everything correctly, so you can't take everything you see in somedit at face value. I do sometimes use it to make mock-ups that I later realize via hand edits.
1) Yes that's the Scorpion Village interiors. Those rooms aren't "unused" as much as they are leftovers. The Scorpion Village interiors use two other town's interiors (Pandora 261 and Kipo 280), overlapped, and just use a select few rooms from the mix of the two. Maybe because the Tropics is so short lived they didn't think it was worth making its own set of interiors from scratch. It also saves some room in the ROM to reuse them, but there was enough unused space that it could've had its own interior.
To see this for yourself, in somedit, go to map 335 and pan to the bottom right; right click and you'll see a context menu which includes:
Piece 61: Edit
Piece 82: Edit
If you pick one of those map piece edit commands, on the right center there's a blue clickable text that says "Find maps that use this piece" and you can see that 61 is used by 261 and 335, and 82 is used by 280 and 335.
All maps are made of 2 or more pieces divided into two layers. Some are made of dozens of pieces. The first piece on each layer dictates the map's size, so some maps have a large but blank piece as their first piece per layer (map piece 235 is the most common blank piece used). One layer controls collision on the map and the other is purely visual, but it's not as simple as foreground and background: both layers can contain foreground and background tiles.
Anyhow, this is how I made the new room for the Resistance Base: Mara's House is map 124 and only uses one room. It's made out of pieces 567 and 568. I overlapped Gold Isle's interior (map 125) map pieces (564 and 565) over the bottom of map 124, and made a new map piece to make adjustments to the room I was going to use.
2) Map 342 uses pieces 18 and 19, which are Potos and Mandala interiors (also note that somedit doesn't make the use of piece 18 easy to find; the right click context menu shows 19 twice). Regardless, there are so many exact same tile arrangements between the pre-release and Potos bar that it surely is the same bar. It's obscured by the HUD, but the pre-release bar also has the south exit in the exact same spot as the Potos bar.
3) By unused doors, do you mean you clicked "Find doors to this map" then checked the resulting list and clicked "Find maps that use this door" and "Find events that use this door"? Not listing a result for 235 is just a failure on somedit's part.
Door235 is used by map 351 (Special event 0xBF type Door value 235).
The other three doors are used by events which somedit does list.
When your character touches what looks like a door, it can either be directly hooked up to a door, or it can be hooked up to an event which can then activate a door. When checking if a door is used or not, you need to check both if they're used directly (by a map trigger tile) and/or used by an event.
A functional door tile is no different than any other trigger tile. It just looks like a door. Assuming its collision data actually includes a trigger. A door tile can also be purely aesthetic.
4) He's not coded differently, somedit just fails to load certain NPC art. There's a green placeholder square in his place.
5) There's not enough data in the ROM to make that determination for certain, but probably not. What you're seeing in somedit is nothing like what happens in-game if you try and load those unused maps. Maps 1 through 14 are or were all probably small test maps for when they were getting collision, animations and layering working. The first real map is 16 and it's not impossible that 16 was where they prototyped stuff.
6) Not sure how you came to that conclusion.
7) Because they reused those map pieces. In this case, they used it as a dead end, so overlapped a small map piece to seal off the north exit. How it looks in somedit isn't exactly how it looks in-game.
8) I think you're just not checking the doors correctly; somedit shows all 3 doors to 128 as in use.
9) Same as 8).
10) Yeah, should've seen how long it took me to make Revisit_Sunken_Continent; editing that map was a bitch!
In somedit, while looking at map 195, upper right is a "State" tab; look at that and enable "Enable map piece filtering" and "Enable object filtering" then flip "Event flag 2" between 1 and 2 to see the map change. It doesn't get the appearance perfect, but it's close enough to get a feel for how it looks in-game.
11) Also super obnoxious to make edits to for Revisit_Pure_Land.
There are 1024 doors. Only 20 are unused. I've used 6 so far in various hacks (1 of them being the Resistance Base door in Reborn). somedit fails to show usage for something like 27 doors that are actually in use.