Unfortunately you are seeing one of the big risks with not choosing your breeder carefully. An owner wanting you to bring (unvaccinated) puppies to them to choose one is pretty much a puppy farmer's wet dream. To be able to sell them at 5 weeks and not pay 3+ weeks of food etc is even better.
The mental development that happens between 5 weeks old and 8 weeks (actually slightly later for Chis) is critical and requires the dog to be within its litter. It is during this time that your puppy would have become used to dogs and learned a great deal about how to interact with other dogs and the world around her, by watching mum. It would have been the time she became confident around other dogs and her world (hopefully).
She has missed this critical part of her development. The reality is that there is nothing that can be done to 100% make up for it because her brain is no longer in the same maleable state it was at the time.
I wonder if because she was so young and because mobility sounds very difficult for your DD whether either if you did any socilisation with her outside classes? This would also have been really important to do (more so, because she was so young when removed from mum).
If she spent the first few weeks within the safe environment of your house, except an hour a week at classes, and didn't go out much then this will have compounded the problem.
I'm sorry and don't mean to make you feel bad. I just wanted to highlight some of the reaons why she might be behaving like this. Coupled with a bad breeder who will not have chosen the parents carefully for health and temperament and ou may have a puppy that is even more prone to nerves because of her (lack of) breeding.
In all honesty, I'd look for a good one-to-one trainer to work with you on trying to gently encourage and incraese her confidence and slowly get her used to the world in the right way.
She's still young so it will be easier to do this now than wait another year or so. However, I would still expect it to be a bit of a long journey and require a fair bit of work...
Sorry.