Oh, and by inspecting Bella they will notice she does not in fact have her Animagus form again, which will cause Dumbledore to rethink some conclusions...

I would be very surprised if Quirrell did not instruct Bellatrix to regain her Animagus form after she had sufficiently recovered from Azkaban. It would not be like him to go to all the trouble to present an alternative explanation for her escape but then fail to follow through.