Definatly feed phytoplankton if you want to keep the feather dusters for any length of time. Our tanks will not support enough free floating/swimming planktonic foods for them without supplementation.
And yes, the crown may regrow a new worm, and the rest of the old worm may regrow a new crown. As long as they can do this without distruption by other critters in the tank. A hermit may find the remains and start in on it before it's had a chance to regrow. Many will drop the crown when stressed (as it sound in your case) and also as a means of reproduction. I've even seen worms split and share a tube for a while before one or both abandon the tube and go off to make a new one.