Not trying to act as authority but hear me out. 

I doubt that it's possible to fix anything as adult with mewing only. Average person who need to fix CFD is already too set back in toung strenght and form needed to get any progress. In my opinion it's only goind downhill from there.

There are some "results" online but i don't think they are real. However there are many progress pictures which clearly show no change.

As adult i think you need agressive methods. MSE+FM could be good option.

@meowxilla

I see, sadly, I don't seem to have access to MSE. What do you think of the normal metal maxillary expanders that are not drilled into the palate?

Also, is the facemask solely for forward growth and upswing and can only work if the suture is split?

@mysterecessed

Sadly i can't recommend anything really as i don't know much i am stil new and diving deep into things. However i am myself using thumbpulling for palatal expansion. I think it has worked for me. Try doing it for 2 weeks and see if you get any changes. In my opinion when you get enough expansion you can try hardmewing it would become easier that way. 

@meowxilla

My tongue seems to fit fine and when I hard mew I get sort of a burning sensation in my palate, maxilla area under my nose and maybe around my ears. Can't keep it up for more than a minute though because the burning sensation feels too strong and achy. 

I have thumb pulled before but I couldn't split the suture. I've heard that some people have done it. Might try it again.