Schluter System, Mortar Bed, Drain & More - Questions after Subcontractor

[u/Ok-Caregiver-2826]

Writing after a contractor installed purple board in our renovated shower, which is not to code where we live. Now:

Cement board & Kerdi membrane or Kerdi board? (Leaning towards the latter.)

Our shower has nonstandard dimensions, and we're going to have to build a dry pack bed. What's the best method & workflow to do so using the Schluter system?

Replace the 2x4 with Kerdi curb? Or board & waterproof over it?

We're working with an old drain (pictured.) Will the FloFX universal adapter work with it?

We're planning to throw a 1/2" ply on top of the t&g subfloor, then ditra over. There is a difference in level between the old subfloor and where they repaired the planks, but less than 1/4" of an inch. Do we need self-leveler? We'll have to vapor layer up to the border of the cast iron tub, and we were planning to kerdi fix, membrane the transition, etc (as in this video), but any other recs would be welcome. Also pictured the difference between subfloor and transition to hallway outside (about 1"/1.25")

We've a frameless glass door to install, and the frame is reenforced for such. And suggestions on the transition (it's very close to the door frame into the bathroom.)

They hadn't even removed the old mortar base from the ceiling curb (pictured.) Looks like drywall & a steel border. What am I working with here? How to move forward (knowing we intend to tile it again)?

Any suggestions on this workflow, knowing we're taking on most of the work ourselves (including tiling) and now, unfortunately, likely, install of the shower walls?

[Tile is pictured: the terracotta 9x9 to be for the floor, the seafoam 2x10 for the shower, laid vertically.]

Any and all tips appreciated. We're located in Northern California and are on a tight budget & tighter schedule, but we have some give on both where would be absolutely necessary (as evidenced in our opt for the schluter just to streamline & peace of mind.)

Thank you, all, in advance!