Event editing: Invisible NPC like in Narshe?

Oh, I didn't even know about another version of the level editor, so far I've been using the "normal" FF6LE Rogue. Would this expansion affect any compatibility with FF3usMe? Because I'm pretty sure I'll still need that one in the future.

No it won't affect FF3usME compatibility. None of the things FF6LE can edit can be edited with FF3usME anyway.

Or maybe some of the unused maps can't display it at all? Just to make sure, are all of the following maps really unused in the game?

That is a possibility. What is that I remember is not all dummied maps can be used, I think those with an underscore can and the others can't or the opposite. Someone else could probably clarify this better than me..