We have a dog that will nip too. Tugger is the most gentle dog with our 3 year old Grandson, and the rest of the family, but he's not as nice with strangers.
What we do, is to keep him on a leash when people come over.....yes, it's a bit extreme, but he usually calms down after about 10 minutes.
Your neighbor was right, when she said that she was at fault....picking up a strange dog is asking for trouble....and then to place the dog in a position of submission (on it's back) is even worse.
I don't think I'd be to worried about Maggie....unless you see additional behaviors that indicate aggressiveness. You may want to have a dog therapist/trainer/behaviorist check her out too....it may be money well spent:)
Good luck