If you had doubled up that stud on the left, the issue would be non-existent. Now the shower is in place, I don't know how you'd attach another stud without taking out more drywall. But in reality, drywall is easy to remove. go ahead and open up the wall on the left to the next stud You can put as many instaback plates as you need for support along the top. 2 between each stud is enough. 3 if you want to go crazy. The patched piece is just as strong as a continuous sheet, as long as you mud and tape it correctly. The correct way is to use hot mud. Force plenty into the seam, place the tape, skim over it to set the tape in the mud. An hour later when the mud is dry, skim over it with another coat of hot mud. I've used these things for over 10 years and never had a problem. It's really the best drywall patch system for small pieces. Anyone who knows drywall, who tries these things, will be sold. Professional time costs money, and these save a bunch of time.

Yes, the screen cloth will cut through both paint and the patch. Don't press too hard with the screen cloth and pad though. Keep the pad flat on the wall and sand the hump down a little at a time, lightening pressure as you cross from patch to drywall ... coarse screen cloth can be aggressive in what it removes and you could end up with frayed drywall paper at the edge of the patch, making the whole thing look even worse. Once you have the patch sanded, you'll probably need to paint the entire wall; otherwise the smoothly sanded patch will show up like a neon sign next to the roller-pebbled painted surface. I'd suggest using a small 3" roller and interior latex primer to seal and prep the patch for standard latex paint. Then paint the entire wall with a standard 9" roller to even out the surface texture.

A good drywaller will remove the construction adhesive and skim coat that nicely in a day. He's not replacing it that quickly or inexpensively.

Looks bad to me. I would hire a reputable drywall company to come in and fix that ceiling right. Obviously, who you had, probably isnt capable of doing any better.