Steve,
That is the million dollar question.
I’ve been there myself, wishing someone would / could articulate what precisely I am supposed to do with my face, etc.
Unfortunately, it’s very difficult to articulate what to do, how to do it, etc. as it’s a combination of coordinated movements involving our lips/face muscles, tongue, air, jaw, etc. In addition, our individual physical composition is different (lips, teeth formation, jawline), mouthpiece, horn, air power / coordination. WindWorks seems to be the best resource of videos and lessons out there to give as much clarity as possible.
The good news is that I think you’re overthinking it, as we all have from time to time…still do, from time to time…
The bottom line is that we need to form an embouchure through which we release air into the mouthpiece/horn to produce a sound. And I know you can do that, thanks to the videos you have shared, etc.
The next step(s) involve changing Shape by engaging the aperture corners (not directly flexing the lip muscles directly, but by flexing the muscles surrounding the lips (the muscles you feel when saying oooohhh).
What precisely to do, how to do it, depends on a number of factors which are very individual. But the clues we need to answer that for ourselves is in our sound, ease of playing, ability to produce a resonant sound, harmonic slurs with speed / ease / efficiency, etc. Can we articulate clearly when using the shape we are using for a pitch?
The biggest problems tend to be clamping down too much top-to-bottom as we go higher and over-blowing, closing off our throat, etc.
Instead, we want to try to employ a less-is-more approach and reduce the aperture by engaging the aperture corners from outside our lip tissue moving inwards horizontally only as much as necessary, keeping our lips as relaxed as possible (so they can vibrate as freely/fastly as possible)
By focusing on what we Can do today, then experimenting with minute changes to use better Process and improve our Results through better Process, we will increase what you can do today and be able to play faster, more resonant, freer, higher, etc.
Don’t worry too much about keeping the initial “aahh ooohh setting”; Greg mentions multiple times that he doesn’t play all over the instrument that way, but it’s a great place to start–as you’re more forward, open, relaxed, less likely to clamp down, etc.
My best experimentation has come from eyes-closed experimentation using steadily released air (to remove air as a variable in pitch change), then making as small a movement as necessary to slot up to the higher pitch. That really helped me hone in on how to change pitch.
The other thing I’ve done a lot of is playing octaves, starting with say G below the staff, followed by G on the staff and (eyes closed) noticing how much difference there is in effort between those notes and imagining that same difference should be what I feel from G on the staff to G above the staff, then playing those notes with a softer higher note with consistently released passive air (the higher note will be naturally softer if you’re using consistent air) and being willing to miss the note. I’ve had the sensation of my lips sort of falling inward naturally–if you blow air fast then slow down / stop, I feel the tissue around my lips kind of settle back down in place towards my teeth. I believe that may be the initial start of it, followed by engaging the muscles surrounding the lip tissues (not the lips themselves, as that deadens the vibrating surface) and cinching the aperture subtly, slightly, only as much as necessary–to allow for the most freely (fast) vibrating lips, resonant sound, etc.
Hope that helps, my $.02 FWIW. I’m on this journey as well, so may have posted something that may be inaccurate that Greg or a more experienced person can correct. I struggle a bit with consistency, but that seems to be gradually improving as well.
