I have seen what you describe in many of my patients. Probably the best approach to to address the dropping of your jaw by using something like a chin strap. It wouldn't have to be tight, just enough to stop the jaw sag. I know Wiredgeorge has found a boil-and-bite mouth guard very helpful in this department. You could try looking at a Simplus mask, but I suspect you would have the same problem.
If you jaw goes slack in addition to your facial muscles, the mask WILL leak as your jaw slides backwards a tad and opens your mouth. I do use a boil & bite mouth guard. This mouth guard is placed in boiling water out of the box and then you stick it in your mouth and bite down to it so it conforms to your bite. It keeps my jaw from going slack and I get almost no leaks. I also use an Amara View with almost no leaks night after night and previously used the Simplus sleeptech mentioned. That thing leaked because it didn't seal perfectly against my face. The leaks were not bad and didn't affect therapy numbers BUT were annoying as air was in my eyes. I then learned mask pads fixed that issue and the mask became very workable. It is just not as comfortable as the Amara View. It is also critical to get the right size cushion and this can take some experimentation. Get a mask and then try a larger or smaller cushion; they are not all that expensive. I am fairly large and the DME techs always think I need a bigger size cushion than I actually do.
I find your last comment interesting and it brings up a good point. There is no correlation between body size and the size of mask required. The mask doesn't fit your whole body, just specific areas on your face. I would have thought almost anyone who deals with CPAP gear would know that. It would appear not. It seems like there are very few places you can go in the US to get an experienced, impartial opinion about a masks or machines. Having said that, there aren't a lot of places you can go in Australia either. I just happen to work at one.
I have not come across any technician who was able to fit a mask. Look at eye glasses... there are three numbers on a frame: aa/bb/ccc where aa is the width, bb is the width of the nose section and ccc is the length of the stems. I find the best fitting glasses are the ones that conform to the width of my nose (the bb number).
Another thing to consider on a mask is the head gear. The Amara View has too thin straps and they bit on the back of my head and neck. My wife makes me little pillow pads to put the the back part. The Simplus head gear is much better designed but the greater surface area of the cushion makes for the most chance to find leaks; especially if bumped a bit at night.
The DME where I was initially "fitted" would not let you actually try a variety of masks for comfort and feel. They decided which mask would be best (I think it was what they had on hand) and then give you that mask as your own to stick on your face in the office. How can any patient know if the mask is comfortable, fits right or is best suited? They can't. I am proactive and figured the mask issue out on my own... you have to keep at it till you find what works for you. What works for me may not work for anyone else in therapy!