I have no idea what the person in the bike shop means by a defective rivet, and if whatever he is talking about is defective it should be replaced. I looked your bike up but have no idea which model you have, there are several different models from that company, I will assume that yours is the Edwin step through frame model. Frankly, you need to find a better bike shop where they can explain what your problem is in a way that you can understand what they are talking about. You do not say what part of the city you live in, there are lots of good bike shops in Montreal and your problem has nothing to do with the Bionix system, any decent bike shop can work on the derailleurs and shifters. I am very familiar with the e drive system on your bike, in fact I just did a Bionix conversion for one of our customers today
Where did you buy the bike?