/\ Good advice, specially about the
avian vet.
Here is your last question.
I once had a Lorikeet that had a blood disorder that must have made its skin itch. It plucked itself to the point of bleeding and actually destroyed the feather follicles, so never regrew feathers. This is rather rare though, If the problem has been diagnosed and fixed.... yes, the feathers usually grow back.
If the bird actually plucks the feather out, they will grow back quickly. If the bird chews the feathers - this often happens - the feathers won't return until the next molt.