I'm no more knowledgeable than plenty of other CIEM-enthusiasts on Head-fi
I do often remind people that individual ear anatomy varies quite dramatically, from person to person, and, furthermore, that the
degree to which one person's ear anatomy
changes, during jaw movement, can also vary quote dramatically by comparison with another person. It seems some people are unfortunate that their ear canal apparently goes through an above-average range of size/shape variation as their jaw moves.
However, as you yourself are contemplating, even these unfortunate individuals may yet find that, when having new ear-impressions done, deliberately holding their jaw and face muscles in a position that is
nearer (not necessarily all the way) to a particular facial position which currently causes them loss-of-seal, may yield a more-successful fit with the next set of CIEM shells.
Quite a lot of the CIEM-related discussion, on head-fi, is geared towards audiophiles and enthusiasts, who generally listen with their jaw closed, save for the occasional yawn or biscuit/sandwich, whereas performers/musicians (especially singers) may move their jaws (and indeed the rest of their body, if dancing, playing an instrument, or jumping-around, on stage) substantially-more, by comparison. Accordingly, performers may find a different approach, such as the above, suits their needs better, in maintaining CIEM seal during a wider range of facial positions.
Please let me know how you get on with the new CIEM shells, after your intentional smiling, when you get your new impressions done. Seriously- I'd be interested to know if it works for your specific needs