Hi, nic. There is really no reason to back prime interior door jams. A primer doesn't stop humidity, it just gives some protection in wet areas.
Sealing the top and bottom of interior doors has proponents and opponents on both sides. If it makes it easier to apply the finish, go ahead. If you are going to finish with the door in the jamb, don't bother. The only exception is that it's a good idea to seal the bottom in potentially wet area like a bathroom or possibly a kitchen. For the same reason, you do want to seal the top and bottom of exterior doors.
For what it's worth, I have a great deal of difficulty with the poly and stain in one products. I know that some people use them with great success---sometimes--but I feel better about the control that staining then painting offers.