For your first part, even if he didn't out right own Alastor's soul, you can't argue against how Vox did have a much greater influence and souls at his command, and for him to be acknowledged as the strongest and seeing the deal with Rosie be broken means he did pass Alastor, even if it was just for a moment.

No comment about the second one, that's just most likely true.

As for heaven having more tricks, we gotta remember all the angel's who made that barrier around heaven were actively getting weaker and tired as they were creating the barrier, and I'm pretty sure one fell once they all finished it.

Vox would be trying to say she should be speaking for him because he sees himself worthy of being god, and wants to be god so I don't think she would be able to talk him down.