Yeah, you've already started this under T1 Eden, that was the ruleset that earned you damage upon Divinity earlier, you can't change the rules now. One prep per turn, everything else is unprepped. 

To reiterate, preps are a functional thing, they do not exist in an IC sense. They're literally just a way of gauging if a defence will match an attack, which doesn't require one to use dice rolls or coin flips. When you're throwing magic around willy-nilly, I see no other method for deciding what magic beats what magic other than the one prep system.