Hi, I had one of these, but actually I had two. It cost more but it was a lot easier having my jaw advanced in two stages, more gentle. So maybe a new one could be made that advances the jaw a little less aggressively? And then when she's used to that she could go back to the current one and it wouldn't hurt so much.
Even with the two, I honestly had to just lie on the couch and watch TV drooling and crying for a couple of days on a school break. I was in college and we weren't trying to make anything grow, but I'd gotten used to clenching my teeth with my lower jaw back somehow and that's what we were trying to fix. But after I really left it in it and got over that initial hump it didn't hurt anymore. It felt annoying, but not painful.
The second one, I went to a massage therapist right away, I had an appointment immediately after I got fitted for the second one, and she worked on my upper back and shoulders and all that area, and I never had any problems adjusting to that one. I don't know if ti is appropriate for a 12 year old to have a massage but if it is it couldn't hurt.
From experience, wearing it 5 hours is not going to make any progress towards getting used to it at all, not the stretch OR the speech. It definitely had a sharp curve of getting used to it. There were initial days of "I cannot do this, it hurts, I can't talk right, I'll never be able to wear this in public," but it really did get to the point where it was no big deal. I hope some of that helps.