I think the Mouse King and Herr Drosselmeyer should be intrinsically linked. That you either can't have one without the other, or that they are parts of some whole. Maybe rivals or even the Mouse King is to his mysteriously leader (if we are fiddling with it, we might as well add characters) the way the Nutcracker is to Herr Drosselmeyer.

I don't think the Mouse King is dead. I see no reason a shoe would kill a mouse, frighten it away maybe, but not kill it.

I've often got weird, creepy vibes when Herr Drosselmeyer and Clara are together that I don't notice when he's with anyone else, so she has to be important. Maybe the Nutcracker was given so she'd have someone to protect. If she was raised to be a damsel in distress, she might need an extra push.

Herr Drosselmeyer fit himself into the family and so Clara sees him as the good guy and the Mouse King as the bad, but what if that is wrong?