Update on Belle's biting/nipping:
First of all, she never does that stuff unless I'm down on the floor with her. And she never bites hard enough to break the skin, but ANY human flesh in her mouth MUST be a no-no.
I tried asdf's trick last night when she went into play/littermate mode. First I got hold of her leash (we leave the leash on inside the house), flipped her on her back, growled pretty snarly, then buried my face in her neck and even shook my head like I was ripping out her throat.
MAJOR TRANSFORMATION. She became even more submissive than she usually is, kinda licked my hand a little bit, and apologized up one side and down the other.
She is a 10-month old puppy still, despite her 58 pound bulk. It will take some time for her to unwind from puppydom, but I'm definitely sure the point hit home last night about biting/nipping.
If not, I'll try deb's technique next!