It is not. It's just that we have no idea.
For questions 1 and 2, try to do a test level and see if it works.
For question 3, yeah, you can make boulders to roll in the sky. Same goes for Ballistikrafts. In fact, some Extreme ROTT levels use this exploit. As for other things, try them.
For question 4, in theory, yes, you can. Extract the floor textures and use them as walls. You can create your own textures and import them back in the .WAD but ensure the colour palette is preserved. In RottED, you can already add the different skies, ceilings and floors textures as "tiles", but I think the game will ignore them or cause a crash in-game. Check the "
ROTT Map Specs" for a bit more details about the WAD file and the walls.
For question 4b, adding a new enemy is possible, but requires: all the sprites of the new enemy, new sound effects, additional foreground tiles IDs and finally, extra coding to handle all that into the game.