CPAP causes Co2 retention / edema / cardiac arrest ????

First of all, unless you're covering (on purpose or accidentally) the CO2 vent holes in the mask, you're not going to be poisoned by CO2... why do you think you are?

Second, there are very many masks out there, in diff. sizes, that you can look at, try on for 30 days (Cpap.com has a huge selection with reviews, explanations, pix, etc.) and if your insce. won't pay for new ones (maybe your DME can let you try diff. ones) you might (?) be able to swing it yourself. And headgear is not expensive in the scheme of things unless you're seriously budget challenged - look at replacements parts on the page of your mask on Cpap.com. You shouldn't be overtightening any mask to fit properly - the cusion needs to inflate for one thing - so keep that in mind and either adjust things or switch sizes - you can't be charged for doing that I hope!

You do sound, however, like you have other physical problems and Cpap can't fix everything of course. If those things are making you particularly anxious, however, it won't do your Cpap experience much good and maybe you need to talk to your orthopedist again if you haven't done so recently.

And lastly, have you downloaded (it's free) Sleepyhead software so you can see many features of what's going on overnight with your sleep - we can help you get set up to do that and post results here so we can advise. You really shouldn't be in such a bad state!

If you are worried about carbon dioxide, the simplest thing to do is to thoroughly clean the vents on your mask.
Do not widen them. They are engineered to be just the right size, as long as they are clean.