Need help on how to restore shower following water leaking past drain cutout from old incorrect installation with no waterproofing [closed]

The ceiling below my master bath shower started to show water damage. I cut out the drywall where it was damaged and saw that the shower drain had been improperly installed (picture 1). I don't know when the shower was modified or installed only that it was done by the previous owner who owned this home for several years prior to my purchase 2 years ago.

I took up the tiling and found that the shower had not been waterproofed. a layer of 1/8" thick mold resistant green backer board was attached to the durarock installed to the wall studs. a 1/4" sheet of ply had been laid on the 1/2" subfloor with similar 1/8" thick backerboard on top of that. The drain hole was rough cut to install the drain with the seal around the drain caulked. (visible in picture 1 and 2).

I have removed all the tiling and durarock along with the 1/4" ply and molded over backer board, scrubbed the wood with borax and let everything dry (picture 3a, 3b). The subfloor is water damaged but not cracked or rotten and is not easily damaged by scraping the wood. I've used a heatgun and scraper to remove the liquid nails that held some of the durarock in place.

I'm not very familiar with custom shower building, but some of what is in place looks uneven or odd. Picture (4a, 4b)

Behind the shower piping is a wall of insulation that is the only seal to a large void in the attic that is covered by a sheet of ply that had insulation laying on top of it. (picture 5).

At this point, I want to restore the shower, but I am a little lost on where to start and how to go about fixing this the best way. I've found a lot of different answers through my searches. One of the big differences in opinion I've seen is about how to waterproof.

Other items I'm focused on are:

• The shower shape is odd, so I think the shower basin will need to be custom.
• I'm also not certain what to do about the attic void behind the shower.
• I intend to replace the mixing valve for the shower as it was leaking past the stem and dripping down the handle.
• This is also the only area in the home I am aware of with partial PEX for plumbing.
• The other side of the shower is an exterior wall. (picture 6)
• I would like to widen the built in shower shelf on the side wall but am not 100% how to do that. I would then remove the built in shower shelf under the shower head which seems easy to just cover on rebuild. -The drain needs to be replaced but the spacing for the drain is already very tight.

Looking for some help to get my head in order and understand the right and best way to take care of this.

Thanks!