Oh... ok. So you wouldn't be able to pass trhough that wall and have a shorcut through the level, would you ? Because some levels have secret and alternate paths I would have thought it was one.
No, there is an invisible wall blocking you from getting over there (which is why you fall down), and then the wall of the room blocks you too.
If you want to get over there, first enter the room and then change RAM address $7E:0C05 to the value $C0 (this is the high-byte of the player's x-coordinate).
Are the red squares indicating enemies' initial locations ? If so, I suppose it's an invisible "enemy" that pushes you down through the sandfall even when you want to go right. This has frustrated me for so many years.
Yeah, the editor doesn't support enemy/object graphics loading yet for RM&F (and is not being worked on by anyone currently, the editor has been worked on by at least three people at this point, and the source code is available on github).
The box that marks the object in the room before the mini-boss is for the stage checkpoint. A flag is set in RAM when the player passes it so you can respawn there if you die.