How frequently do the zombies drop keys

They are dropped. The time between drops seemed random and sometimes too long IMO, but i got keys for every gate except for the ones in the 3rd world (still playing...)

Since they come in sets of three, the Bloomerang does quite well; it hits all 3 parts of the camel and then all 3 of the zombies after breaking the screens, and it does so twice. Bloomerangs are less effective if there is another zombie in the way though, since they will only hit up to 3 zombies before returning.

I haven't gotten very far so I can't speak for the other plants, but you'll have the Bloomerang before you start seeing the camel zombies, so they're a solid early game choice. Logically speaking, any plants that penetrate or deal area of effect damage should do similarly well; the risk with these zombies is if you only damage the front they can easily overwhelm light or modest defenses.