A left inswing is a RIGHT in outswing.

Pics aren't loading. It sounds like the installers had no clue how to nail to exsisting ceiling joists and probably just nailed it with Brad nails to the sheetrock. Brad nails would work but they have to hit on the exsisting structure, After they fixed the sag in the ceiling. And if they glued the new ceiling, don't let them back on the job. Shortcutting your job now shows what the rest of the job will be like.

You can tile outside of the shower over sheetrock. You need Hardie or cement board in the shower for all walls. Silicon all seams and use a trowel to flatten them. The three coats of redgaurd over the entire shower. And the seam where the sheetrock begins. Then tile over.

But would it gather moss if it stayed?

They make a poly fitting but they are limited in functionality. You could use a poly coupler, transition to pex then use a sharkbite

Whale milk is considerably different from cow milk

Ex wife did too, hence the "EX"