The wavefunction is the same object shared by all universes, correct? Thus a point's spatial neighborhood in one universe is not the full neighborhood of the point. I would imagine (if it's a coherent notion) taking a derivate only "within one universe" would have a different result than taking it with the full wavefunction.

I meant the universe's neighborhood, at taking the derivative of the universe's wavefunction at that point.

Wouldn't an air molecule already going one way need a separate cause to be going that way (as in something that pushes it that way (probably another air molecule))?

Since the wave function is continuous, if you look at a universe with a particle nudged just a little bit, the wave function won't change much. It's not like you're moving that particle very far.

I guess any device that could detect the scrambling would be as good as detecting the particle directly?

No. If the air only ended up in that orientation if the particle went in a particular direction, then the system would decohere, and the detector would be unnecessary. Since the air can end up in the same orientation either way, there's no way to detect it.