If the nights have major differences in setting and gameplay then sure (something like Sister Location comes to mind). But if it's more like my fangame (which is just like FNAF 1) where pretty much the only difference between nights is the "levels" of the enemies, then probably not.
uh honestly I dunno. Could be a number of reasons, maybe you've got a lot of stuff in the scene, or maybe a hardware thing. In my experience it takes a lot to get Godot to lag badly
if you make all the nights in one scene, how do you make to the player only play one night if he finish the night before? also, how do you make to keep on track the night that the player is in?
Assuming that the only difference between nights is the difficulty of the enemies, all you have to do is change a variable containing those difficulties. You can use a global variable to keep track of which night it is, I explain how to do that in the intro of this post.