My doc agreed with most of what Mew says, but the real question here is about palatal expansion in adults. And it makes total sense that IF your palate plates have FUSED, you will just push your teeth out, which will sort of give you a wider smile, but might as well cause issues and won't give you the jaw expansion you need for aesthetics.
She showed me an actual example how in people who have deformed, uber narrow jaws and are above age of 25 or so they saw the palate in 6 parts basically. Like 6 little blocks. Then they can slowly, with palate expanders completely transform the jaw into anything.
This ortho is also against extractions. She also says that muscles play a big role into shaping of bone tissue.
The thing here you have to understand are the forces in play. You need to exert MORE force on the bone that is being used to holding it together. Can you achieve that just by holding your tongue properly? I'm not sure...
I'd really like to see actual before and afters where adults have achieved significant changes with no braces and no surgery.
I got braces and it improved my bite and my face got more aesthetic, but it was due to braces and bite, not due to change in palate.
"You need braces and surgery!" - RUN! See Orthotropics working on people everywhere of all ages
Both of these are extremely effective in most cases. Especially if you avoid extractions. There are cases of literal subhuman looking creatures transform into models because of double jaw surgery.