If I understand this correctly a belted cartridge seats to the lip of the belt inside the chamber. This being the case when shooting new brass the shoulder may not be completely seated in the chamber.

After fire forming the case has the same "custom" fit as a non belted case. Is this right?

If so, when using a belted case that has been fire formed if you just neck size it then you will have a formed case from there on out- yes?