I suspected the same thing as you–my teeth meet in the middle, I have no overbite or underbite; perhaps that’s the reason I couldn’t play high. My top lip is kind of thin, perhaps there’s not enough meat to oscillate, etc., etc.
I feel like I’ve disproven that to myself now; while I’m still just an amateur hobby player with limited skill, the sensations I have and the range I’ve developed in a relatively short period of time lead me to believe I’m on the right track.
There appear to be great trumpet players with different types of lips; some big some small. Some play on big mouthpieces, some play on small.
From what you’ve posted thus far, I suspect you simply haven’t had that “coffee moment” where you feel the sensations of more (much) easily doing harmonic slurs simply by engaging the aperture corners from the sides, letting the middle of the top lip be relaxed, using a less is more approach. I think if you experiment with that, reading all the Windworks pointers before closing your eyes and experimenting with harmonic slurs, I think you’ll experience (eventually) an ease of playing you didn’t think possible before and with minimal movement, just an engagement of the corners / slight contracting as you ascend.
If I were you, I would take Greg up on his Skype offer. It’s most likely not an easy fix, but I bet you would get some good take aways from the session that would help you on your way.
So much,, probably most, of this is mental; you need a good morale and frame of mind when you practice or you won’t progress.
Be patient, open-minded with no expectations, be willing to experiment trying to do what is described and be willing to fail and realize what you tried didn’t work.
Focus on the Process, not Results. I feel like I’m still solidifying the Process but I no longer fear losing it as much as I used to. It seems that it’s become more second nature now than my old way of playing; that takes time and will likely take me longer to really cement in. Good luck!