#morph allergies

I think with the laws of morphing as written, it wouldn't be possible. The biggest symptom of morph allergies is an inability to stop morphing, especially when you don't want to morph. You have to remain very calm to avoid morphing, and nothing is more likely to sabotage that than the knowledge the two-hour limit is approaching.

Yeerk might also go splat, depending on how large the animal is and how suddenly it appears. Human skulls are pretty durable, even if hork-bajir seem to be a little more delicate and a little faster-healing. Yeerk + squirrel inside of human skull = yeerk and host brain go splat, skull itself is fine, squirrel is having a real bad day.

OOooooohh, finally a good idea to come out of this horrible cursed headcanon. It's not 100% clear if you are allergic to, say, that one particular crocodile or to crocodiles in general. But assuming that it'd work with multiple crocodiles (or condors, or otters, or whatever) then you'd need one heck of a selfless morpher willing to endure several days of morph "burping" followed by an unpleasant morph "barf" to get a good population going. Still, it could be like donating blood: you could sign up to do it once every several months and get several donors going at the same time to create a steady supply.