I could be wrong, but you might be taking the MmmmwwwwaaaaOooohhh setup thing too literally. My take on it was that was to get us setup to put the mouthpiece on our lips and have the sensation of an open throat, easy air and engaged corners.
My thoughts entirely. Maybe Greg can chime in here but having look at the videos again my take is that the MmmmmaaaOoooohhhh exercise is just getting a player who plays lips clamped into feeling a new sensation. A sensation that will feel very odd because the centre of the lips are relaxed and open. However I don’t think Gregg says anywhere that you play with such a forward pucker. Certainly I cannot. However if I put my lips together touching ever so lightly and then let the centre of the lips open a little, where I can feel the lips still together are the aperture corners. If I then engage these corners by pressing the corners ever so slightly together and moving them inwards, I have my playing setup. Engaged corners, centre of the lips apart and relaxed but no pucker.