I don't know how you feel about wearing a cervical collar to help support the chin and prevent the jaw drop from opening the mouth but some people find it helpful and not too annoying.
If it were me I would probably just tape my mouth shut...but some people get freaked out by that idea. I taped for about 3 months when I first started cpap therapy because of mouth opening issues mostly from habit of the mouth breathing gasping for air because of the apnea events...my nose was never the issue in terms of congestion causing the mouth breathing because it was always clear.
If the leaks weren't horribly excessive and didn't wake you up....I would probably say just ignore them but if they are disturbing sleep then I would want to fix them.
I know someone on another forum to tried a rather unique approach to covering the mouth but not a 100% seal like you get with the taping option.
See what Jay says about using the Scunci
http://www.cpaptalk.com/viewtopic.php?f ... i#p1086296If it were me in your situation and your experiences with a full face mask...I think I would back up and try to fix the mouth opening issues and use a nasal mask..
I can't think of a full face mask that you haven't tried that would be a potential better option.