Fortunately I've had 5 years of incredibly good service from my own HW100S, but if I'd parted with the thick end of 700 notes and had problems as described, I would have been demanding a replacement rather than a fix, either that or a full refund.
As said, I'd be getting in touch with Weihrauch, explain the problems and how dissappointed you are with such a high quality gun going wrong and look for some kind of recompense.
They do have a reputation to uphold and the germans take their reputations very seriously.
I'm sure this is the best course of action.
All the best
Duncan.