I'm not a gun expert either, but it seems as though the chamber needs to be polished at a minimum and may even have to be re-cut( if possible). I think the chamber has a rough spot causing the dent and I suspect the throat needs to be re cut also. The neck of the case is being compressed when chambering a round and brought back to the proper dimension when you run it through your neck expander plug in your die. Since the work I've described can only be properly done with the barrel out of the receiver the rust issue can be resolved then also. You have a VERY rough chamber and a throat that ,I believe, has been cut too short or is undersized. I would take it to an experienced smith. That work should have never left the shop. I know what I've said doesn't make you feel good but that throat may even cause excessive pressure when a round is fired so best have it fixed.
good luck and good hunting
