Cia, thanks for the clarification on the biting and mouthing. It takes a while, as you now know after reading all of those threads, to get to the heart of things, and I appreciate your patience, and good sense of humour.
I think when you made a loud yelp, it was sincere and she learned from it, which is exactly how it should be. However, if your BF is encouraging rough biting, that is counterproductive and confusing for her. Stand your ground with him as you are right! Tell him I said so.
If you do catch her in the act, use a word that you will also use to indicate a need to go outside. For example, I always "ask" my dogs if they "need to go out?" because as pups, I always used the word out, or outside, while exiting the door, with loads of praise.
Once out there, you can teach "go pee pee" or whatever suits you. I personally don't like "go pee pee" as it's rude in public. I just call it all "potty". Every dog I've raised has learned the word "outside?" or "out?" and they do learn to actually answer the question. That takes a little while, but she will start to understand the gist of it. Although we instinctively want to shout "NO" when we see the puppy piddling on the carpet, take a deep breath and in a happy voice, say "Outside", or "Out we go". Subsequently, each time you take her out, say the same thing in the same tone of voice. You create a command essentially, but one that is positive.
Phyrie's time lines on understanding and comprehension are spot on. Just when we think they've learned something, we discover they haven't and realize that it was us humans who had learned the pattern.
Brenda is right in that we are a straight forward bunch of people. Although you mentioned the answers didn't seem so straightforward, I think you'll see a consistency of training ideas, that work and are positive. Every puppy is unique, like raising children, so there is no "one answer", merely guidelines.
You have the right idea about everything. Trust yourself, and your instincts. You are already a wonderful Doggy Mum.