I agree, buzzing can do a lot. A clear
Kelly mouthpiece can show a lot too!
I think a great way to help would be with a set of
Chop-Sticks embouchure training weights. These weighted sticks have a twelve week program that trains the embouchure muscles. It will work the embouchure AWAY from the mouthpiece which is good since it is a temptation to go back to the old way.
It also has an advantage that the instructor can see the embouchure. Using these weights as stated in their manual, the the embouchure will develop nicely and correctly. I have used them personally with great success.
M&C