Actually, the switch is getting SpiritSphere DX tomorrow! (an enhanced version of the PC version). This version will be exclusive to switch for a while, but will probably make it towards the PC version at some point in the future).
1: This is not possible right now, but it's there in the DX version.
2: The DX version has character select, but no 2vs2 for AI. The DX version does have a 3 player mode, in case you have trouble finding a 4th player.
3: Not possible right now, but once again, available in the DX version.
4: That's the point of the sphere though! Paying close attention to which one the real one is.
5: Yeah of course, go right ahead!
Sorry that I have to tease you with all the DX version features, but it's a big addon to the base game. I hope you will still have fun with this version though!