Which mask are you currently using, and which have you tried? It is a good idea to get a professional to help you with mask fit and adjustment when you first get it, as this can make a difference. My personal favourite full face masks are the Fisher & Paykel Simplus and the Phillips/Respironics Amara View. Getting the right mask and the right size can often mean that it doesn't need to be as tight or uncomfortable, and also remember that a little leak isn't the end of the world if you sleep better. Have you considered a nasal mask and chin strap? Plenty of people with high pressures (such as yours) still use nasal, and it would help with your dry mouth. Another tactic to lower the pressure (and thus improve mask fit and decrease dryness) is to find a way of staying on your side while you sleep, because some people can get away with a lower pressure while on their side. Are you sure that you are using you humidifier correctly? A lot of my patients don't.
When I first was diagnosed and prescribed, I received an Amara View. I hated it. IT LEAKED. I bought a Simplus on my own dime and while it leaked, I started using mask pads and it was not irritating as that stopped air from blowing in my eyes. I then experimented on my own dime and bought one size smaller mask for the Simplus and it was a revelation. While I still needed to use the mask pads as it doesn't seal totally, leak rate fell to under 10 L/min which isn't real significant. For the heck of it, I bought an Amara View cushion one size smaller and BINGO, it works wonderfully with no leaks most nights. The Simplus has a better head gear. No mods needed. The Amara View head gear side straps hit my ears and begin to hurt and the small area at the back of my neck starts to itch and is truly annoying after an hour or two. Fix was to have the missus sewe some padding on these areas and now the mask is tolerable. Could be better but works well enough. Leaks with a full mask can happen if you have your mouth open and the mask slides up over you mouth since this condition is caused by a slack jaw. I use a boil and bit mouth gaurd and no more jaw sag or open mouth. You will figure things out. The size/shape of your face may seem one way to your (I am fairly large) but you may end up with a small mask cushion as I did to make the mask fit your particular face. Good luck.
I also have a small face and cannot use a full face mask, though I have tried several. My machine is titrated to 20 with a pressure relief of 3.
If you are using a nasal mask, try using a chin strap in conjunction with taping your mouth with a strip of medical tape (Nexcare Durable cloth). You can find it at Walgreens and sometimes at Walmart. Some people use paper tape, but I could never get that to stay on. Make sure you fold the ends over about an 1/8 of an inch so it's easy to remove in the morning.
To help with the chipmunk cheeks, you may want to cut down a toenail separator (Walgreens) and place a piece under each of the mask straps and against your cheeks. I put a small make up round underneath each piece of the separator so they don't leave marks on my face. This will put slight pressure on your cheeks so that they will not puff out. Finally, make sure you use the ramp (at least 10 minutes) on your machine. The chipmunk cheeks will be even worse if you are not using the ramp.
I have had the same difficulties you describe and am finding that over time, my brain/body is learning what to do to prevent all that air from coming into my mouth, but it has taken a few months.
Xylamelts (CVS Pharmacy) are also very helpful with dry mouth. You can even put one on either side of your mouth between your cheek and gum line. It dissolves slowly as you sleep.
Hope that helps, and good luck.