'Breeder' stated she did not know how long before she weaned the puppies.
You put 'breeder' in quotation marks. Indeed a 'breeder' not a real knowledgeable breeder. More like a puppy farmer.
Yes to exercise, but not too much. A pup should only get gentle exercise until the bones and muscles are firmed up.
Try ignoring her when she is having a hissy fit. Sometimes it is only attention seeking. I like Redial's suggestion of chastising her like her dam would have done. You aare going to have to reinforce good behaviour (mostly with voice and cuddles but also with (rare) treats) and let her know you are unimpressed by biting and that that will get her nothing, not even attention.
Try to find a trainer who has experience with Basenjis - Google can give you generalities but experience is better than reading.