Bathroom ceiling absorbs moisture and falls apart
I think at this point it's safe to say the drywall above the shower is shot, and now you're going to need to replace it all.  While you're in there, install a proper exhaust fan.  Yes, I know that's exactly what you don't want, and that you're trying to cheap this out, but that hasn't worked, and you've given "cheap" the full tryout.  All "cheap" has done is nickel and dimed you to death.  

Working with this bathroom as-it-is would require very different bathroom discipline.  It might work for other people.  It doesn't work for you (or other family members). You just don't have the patience.  Get the exhaust fan.