Bendable blends?

So, blends have come up here a few times lately, most recently with regard to frenemy, and along the way Stumblerette was prompted to ask some good questions:

Why is a schnoddle (schnauzer + poodle) not called a pauzer? Or a labradoodle (labrador + poodle) not a poobrador? Or a cockapoo (cocker spaniel + poodle) not a pockle? (To my ears cockapoo doesn't sound like anything you'd want to cuddle, but that's just me.)

I know there are ideas out there about the ordering of elements in blends, but others know infinitely more about that (and I hope one or another of them will comment it). And for the record, pockle sounds about right for those little things.

But yesterday brought an instance of a possibly reversible blend: A colleague was telling me about a bus traveling the country or at least the region with a historical exhibit. (That's another story.) She said "It's called the museobus or something like that. Turns out, she passed along later, it's called the buseum. But google these terms and you get results for both, but museobus almost only for French. If you'd asked me, I would have bought museobus, but never buseum.

So, morphology buffs, what the heck is going on here?