I did post a few weeks ago about jaw position. In the upper register I find myself pushing the jaw forward with a pivot like movement. When doing my Ruby exercises I get clear soft notes but cut out on the crescendo but sometimes opening up with this jaw movement. At a gig on Sunday I struggled with a G at the top of the stave but made this adjustment and found a clear sound enabling me to play much higher. This does not always resolve this way and sometimes no clear sounds emerge. When I get a good register and do not make this adjustment it seems a more correct way of playing but still seizes up sometimes.
I realise this is a problem and I have been experimenting by going back to basics and doing the Largo exercises daily with as much purity of process as I can to try to solve it. I then do my Ruby exercises trying to keep the feeling of the Largo and have started to dip into Emerald. An aha moment occurred in Emerald when you say you are using KMT tongue. I tried that and I noticed a big difference.
I warm up each day with your extended lip bends, articulation and long note study. I am getting to feel the tongue position having a dramatic effect. I work with choirs and emphasise tongue position behind the bottom teeth on open vowels to get a more relaxed feel and larger cavity for resonance. It is the most natural position. I am now finding this lower tongue position pushing the jaw slightly more forward in the lower register making the the transition to the upper register easier. It feels unnatural after so many years of tonguing behind the top teeth.
Without you seeing what is going on are able to assess if this is the correct approach?
However, the lower lip is slowly changing position from tucked under to the shoo position. I am feeling more and more the aperture movement. I have been doing this since the course was launched and still feel miles away from your ideals but my playing is improving and people are noticing.