...I can't help you with your question about the CPAP, but I can tell you that I try hard to sleep on my back when I am treating my face and chest. I don't always stay that way, but I can usually stay on my back long enough for the cream to absorb before I end up on my side. That seems to help keep the cream from getting in my eyes. I place pillows on either side to keep from rolling over as easily.
Have you tried applying the cream a few hours before bed so it absorbs prior to using the CPAP? This might keep it from being affected by the straps and, hopefully, keep it out of your eyes as well.
I am hoping you are able to find a way to get some rest that is better on your back than a chair. I know you are bound to be exhausted at this point.
I am including a couple of articles you might find useful as you move forward.
https://skincancer.net/life-with-skin-cancer/tips-for-efudex-users
https://skincancer.net/life-with-skin-cancer/efudex-side-effects
I am sure there are others who are dealing with a CPAP during treatment. I would like to suggest a Facebook support group for Efudex users. I have been a member since 2014, and it is a great source of advice and tips. I am including the link here to join the group in case you are interested in joining and posting your question there as well.
https://www.facebook.com/groups/191070307590515
I am wishing you all the best as you continue to treat. I hope you get some wonderful results!
We are here for you!
April, SkinCancer.net, Moderator