In answer to your questions:
A hen will incubate them nicely. Only problem is the chicks don't understand hen language so won't come when she calls. Therefore you need to make your run chick proof (ie very fine wire, or keep them in a shed).
If they can get out, because they don't respond to her calling, they will wonder off and die of starvation, cold or predation.
Feed them on pheasant or turkey chick crumbs which contain an anti blackhead medication and are higher in protein than poultry feed.
They don't carry things leathal to your chickens - more the other way round. So if possible rear them in an area that hasn't had poultry on, and worm the mother hen before putting her on the eggs.