TMD can give you cracked teeth and lopsided jaws without mewing. Vaulted palates can be helped with Mewing, and appliances may make the process faster and easier. One of the goals of the use of the ALF appliance is a flat palate, and it's just a very simple device that supports the work of the tongue. Maybe if your tongue was shaped like a cigar your palate would become more vaulted with mewing lol but I think most people with narrow, vaulted palates will have their tongue pressing against their teeth on both sides when their tongue is on the roof of the mouth. Mewing has been the remedy for a narrow and vaulted palate for me. I don't understand how my palate dropped down in the back... if that was a result of  posture correction, mewing or some other factor. But I believe I recall this was written about before, so maybe someone will post about how that works.

@darkindigo Not sure what you mean about mandibular teeth being trapped? Is that when the upper arch expands and the lower arch doesn't? My teeth still meet, the lower arch has expanded a bit (though I never thought to measure it). Yes, been mewing for several years and progress is slow overall, though at one point progress sped up considerably and teeth still meet. I get the impression you'd need to expand really fast with an appliance to encounter this problem.

From my experience my upper U got wider and a few weeks later my lower U followed-up. Once upper U is also wider than lower U mewing properly will feel more in the lower U than the upper U. So I'd say mewing expands both at the same time, as long as it's done with the posterior part of the tongue mewing. 

The palatal vault flattens while it is expanded.  Depending on the cephalometric reference points, it either seems to come down or go up, so these are all relative movements rather than absolute ones.