I dunno, if you want feedback so much.. even now.. umm..
overtones.. well, someone feel free to correct me.. you get overtones when you have more than one voice going, and they are so in tune with each other that their soundwaves lock that you can hear notes (the overtone series) sort of shimmering above the actual notes you're playing.. an individual can't "play" overtones..
as for the airiness part, you made a good note on using your diaphragm. sound problems usually have more to do technique and air than they do with mouthpiece sizes..
not quite sure what you're talking about in changing your embouchure
"when halfway though a long, low tone session" to create a better sound, but um, just always have it set correctly. changing your lips also tends to change your pitch or its tuning, so just make sure it stays steady and consistent.
ok. if you need any more feedback or anything, please feel free to let me know. I'm having trouble sleeping tonight so I'm just going through the forums I usually never read, but ah, PM me and we can chat..