If you don't mind sharing, baa, what is your apnea diagnosis, and what was the prescription? Most failures in PAP trials are due to fit, personality of the user, or the wrong prescription. Most can be fixed, although the personality bit is a real uphill slog. Have you ever placed your case before the good people at apneaboard.com? They run a really tight ship there (I used to be a monitor), and they have some highly knowledgeable members on staff who are good at figuring out how to make PAP therapies work for their 'owners.'

If you have a modestly high probability of seizures, you should probably not sleep upright, but if you really must, simply place large foam sheets, maybe 4" thick at a minimum, around on the floor around your bed. If you fall sideways, those mats. about six feet long and at least three feet wide, should prevent you from much harm. Place the mats closer to the pillow, and you can forget what happens to your feet.... mostly.

Sorry, that's all I have to offer. No experience with this, but this would be my first resort if I were faced with the same problems.