Patty, in reality, mouthy adults learn much the way of puppies. By mouthy I don't mean aggressive, but simply using their mouth to play bite. Yipping, ignoring, or holding mouth closed and teach the "close your mouth" command in that order. Many stop with the yip and ignore. Some you just have to teach the close command before they get it.
I totally agree learning with the litter and mom are the very best. In fact, although we rarely have information on adult rescues in rehab, I would bet many were taken young from the litter. Many of the pet owners I have worked with absolutely got puppies taken too young from the mom. It's the cycle… irresponsible breeders placing too young, owners who don't know enough to find good breeders and also don't know how to teach bite inhibition. But some of course are orphaned pups or from shelters. I am glad some states make placing before 8 weeks illegal, but sadly all the owner has to do is lie about the age.
With adults, in your situation, I would tend to teach "close your mouth" as opposed to just a soft mouth, btw. Your dog had bite inhibition, just needs to relearn it.
Long ago Kathy Diamond Davis and I discussed the issue with my OWN Rottie who simply could not, would not, keep her mouth off her family (no one else). Excited? Wrapped her mouth around a body part. Never the slightest bit of tooth pressure. Her soft mouth was fine for us, but what if she, a 90 pound dog, did it to someone else, even without hurting them? Anyway, Kathy's method worked for her most of the time if we saw her getting revved up and used the "CLOSE IT" command.
Kathy is a friend, and you can contact her if you have issues, just tell her I sent you. LOL, I like the hard dogs, but Kathy is super with fearful and timid ones. She also helped save a raving maniac Lhasa that I couldn't bear to put down (severe abuse) but also wasn't happy getting mauled by. When upset, Hannah would charge and bite. Kathy said "get a laundry basket, drop it on her mid charge and give her time to think about it without the fear of you trying to physically stop her." It was miraculous. In about 5 times, the charging stopped with a command, eventually didn't even begin. We placed her with a friend where she got to live out her life in peace.
I will never take another Lhasa, btw. 6 mos of prozac to get her over fear of night, almost a year before she was ready to place even in a protected environment that could absolutely assure no child would ever be near her. I won't bore with details beyond that but the rescue group that asked me to rehab her is still apologizing. (they didn't admit til after I took her that Hannah was short for "hannibell Lecter.") And yes, normally such a dog would have been put to sleep by me in a flash, but her history and her incredible sweetness at other times made it so hard. Even my vet who wanted me to put her down, when she was hospitalized for 2 wks for an eye infection (in her adoption home by then) admitted she was very glad I didn't.