Nope, imagination is a function of an creative actor and so presumes the latter's existence. I actually did give this some thought after you posted this. If the premise is accepted that Tommy Westphall can imagine himself into existence using his god-like powers, then that makes analysis impossible, and an idea that makes analysis impossible is necessarily an idea that doesn't make sense and should be discredited because of that.

The reason analysis becomes impossible is because if we accept the premise that there is a god-like creature who is imagining it then what's to say that it's even Tommy Westphall to begin with? Why can't it be some other entity that is projecting these ideas into Tommy's mind? With an idea that vague we might as well ascribe them to the "writer" in which case we're left at a maze of circular logic. Without a distinct notion of exactly who the source of the ideas are, we have to discard the idea as "could be true, but if it is we would have no way of knowing" and proceed without your and Squirrelson's premise.

That does raise another possibility though, it's possible that, yes it is a multiverse (still unproven AFAIK) but the Snow Globe operates as some sort of mystical Lovecraftian device that allows Tommy to momentarily see through the veil that is pulled over everyone else's eyes. This may be a competing theory to the original premise (Westphallian creation theory versus Westphallian perception theory). But I would have to think about that one for a while to see if both theories at least start holding water. The perception theory is of course predicated on the assumption that it is indeed a multiverse.

Although now that I type it out, the perception theory could exist within a simple universe. He could himself reside in a multiverse and is using the Snow Globe's mystical powers to perceive an entirely different parallel universe.

It's food for thought I guess, but I'll have to add it to my notes and proceed from the idea that there may be two competing theories.