In the case where the gap is too small I suppose I could take your approach and caulk the gap I am just not sure what the maximum gap size is that caulk can feasibly cover. Though the stretch percentage numbers look good on the caulking if you work out the actual nominal stretch sizes it works out to be quite small. On a 1/8″ gap even if the caulk can stretch to 200% that means if the wood shifts by 1/8″ then the caulking fails.

What caulking do you favor for sealing bottom plates. In my climate the preferred method for this is to use an acoustic caulking to tie the vapor barrier which is on the inside to the floor/ceiling where the acoustic sealant never really dries but stays in a sort of gummy state. I am not a big fan of this and caulking in addition seems like a sensible step.

I also wonder about spray foam and how it handles thermal expansion of wood members once the foam sets it is unlikely to have any stretch.