You don't need a devilsaur at all if you're doing a minimalist list of pets, since you already bring a mortal blows effect yourself as a hunter.
BM hunters looking for that debuff can bring a rhino, worm, ravager, or boar.
The 'any' for BM for physical vulnerability means that you can bring any of the four physical debuff pets, as opposed to just the two listed for the other two. The spirit beast heal isn't as big a benefit as all that. The cat appears with the spirit beast in the BM column because you could technically run with either and it wouldn't matter. I didn't list them only because it would have made the chart even wider. You're correct that quilen should be listed with water striders for the crit buff. So, to address the rest more specifically: That's why there are BM-only pets in the last column - they're representing the BM-only abilities. The 'both' column lists the fewest number of pets you need to cover both BM and a non-BM spec. The compact taming list is the smallest number of pets you need in each spec.
I think a lot of your questions will be answered by the following points, Rawr:
