Unfortunately - as you know - this is a hot mess. How did the waterproofing happen around the deck? I’m afraid any water build up (even just while showering assuming you towel up right after) could cause problems around that whole perimeter. Aside from switching from showering to bathing I’d say this is a redo (because even with a shower curtain you will have water build up at the fixture wall side while showering).

The mistake is fatal. You need an alcove tub with a tiling flange. Then redo it all.

Let me ask the question in a different way. If the bathroom has no standard 60" dimensions, how does one install a shower/tub combination of any kind?

Flanged tubs are all 60", with flanges on all sides... my bathroom is 65" wide... just not wide enough even for a 66" tub, yet too wide for a 60" one.

You had space for a proper alcove tub so that is what you needed . Now it is really going to be almost impossible to control the water usually I like a hinged glass shower screen instad of a curtain but I think now you will need a curtain that is long enough to go inside the tub and all the lengy and around the corner opposite the shower head , not the most attractive look but functional. BTW next time come here first since the textured tile is not IMO a good choice either . I do wonder what you did for waterproofing so maybe share that here please before you have bigger issues.

Some version of this.

If you had to retain most of what you have done, you could remove the showerhead/accessories, repair that tile wall, and build a stand-alone shower somewhere else in the bathroom. But that's probably not in the cards.

Hard and expensive pill to swallow, but just bite the bullet and redo this. Lots of people lose money on things: stocks, gambling, . . . . bathroom remodels gone wrong.

I think the plan will be to replace the tub with an alcove version of Underscore, (60x30) without a front apron, pushed against the wall on the water wall side and the back, and create some sort of build-up on the right hand wall. I would still put a wide front to the 36" width I currently have in my deck, but since now my deck is only visible from the front, I might use a single-piece of marble to line that front, similar to the bottom of the niche. I do want to maintain the width of the alcove (this bathroom is for us, and we do not have mobility issues), nor do we plan to sell the house soon. The only tubs with slotted drains I could find are Kohler Underscore or Archer and Hydrosystem Sydney, which is built on demand, but might be able to conform the space for both width and length. It has nice wide, flat rims, so I would not need to fiddle with a deck. Not sure if it would work with the Kohler version of the slotted drain, or if they make that drain cover in polished nickel, so there is some research to do.

To answer questions about moisture barrier, the contractor installed Schluter membrane on cement wall under the tile, on walls as well as on the tub surround. The current tub then rests on the tile, with caulking around it. Is there a better system for drop-in tubs, especially the treatment where the tub meets the deck? Or is there simply an assumption that a soaking tub will not splash water over the edge? (I can't imagine kids in a tub not splashing...) We do have a shower elsewhere in the house, but not in this smallish bathroom. This really needs the double function of tub and shower.

Also, other than esthetics, is there a structural reason why the 3D tile would not be appropriate in this application?

Was the 65 inch measurement stud to stud?

What kind of tub did you have in that space before?

There was some reconfiguration. We had a closet where the vanity is, then a makeup/sink area, then a tiny bathroom with a shower. We turned the old shower into the new closet and connected the rest of the bathroom to the makeup area and closet, so that we now have a bigger room with a decent size window for natural light. So to answer your question, the alcove sits where the old tiny bathroom was -- same width. There was a toilet and small vanity before, the old shower was walled off into a needed closet. Mid-century split level home, not a lot of space to work with. This is the tiny bathroom:

The width of the bathroom is difficult to change, as the interior wall separates the bedroom and hallway. We would end up with a crooked wall and hardwood floors to rip and replace.

Since your space is less than the the 66 inch stud to stud space needed for the longer tub, I would just frame in the area to 60 inch depth needed for a 60 inch tub.

Do you really need the tub? Can you tear out the tub and make it a walk in shower?

I would still put a wide front to the 36" width

Those wide front aprons are brutal & dangerous, IMO, to straddle getting in & out of the shower.

Safety wise that is an accident waiting to happen.

Sorry total layman not getting the problem. Our shower current wraps around a bit at the end and would stop any overspill of water. Pending that not working good old Home Depot has very basic corner pieces you can install.