I just blew my knee and the other is hurting now, so I think your walk in is an excellent way to go, and I would add making it handicap accessable because it is easier to have it accessable than trying to remodel after a need arises.Have a no step floor that a chair may roll in easily. If you want to redo, why not do for many possibilities?

I like shower niches. Very useful for all the items that otherwise don't really have a home. But I never understood why niches are just a recess in the wall. Things would always fall out. When we did our shower, we told the contractor to install a shelf in the niche, a couple of inches from the bottom. The shelf is made from aluminum sheet metal and we had somebody cut out different sized holes with a water jet. Now, bottles, brushes, toothbrushes ... all stand savely supported and can't accidentally fall down. We made the niche extra tall and put a second solid shelf closer to the top. That's for items that have a flat bottom and that don't get used frequently. Total cost was a few hundred dollars for the water jet work and the extra labor by our tile setter. But so much more functional. Enjoy it every day.

That's a glass-mat backer board. If you haven't already been there, there are installation instructions on the manufacturer's website that will offer installation instructions for hanging the board as well as detailing the panel seams, corner joints, and fastener holes. For your tub, I don't see them in the photos...but the tub should have shipped with flange clips. For Duravit tubs, the bottom part of the clip typically overlaps and covers a the tub flange. If I'm reading your photo correctly and that's the tub flange showing below the backer board, I don't see any clips showing in your last photo. After the tub is leveled, the clips are used to secure the flange to the wall framing. Some tub manufacturers recommend filling the tub with water so the tub is settled. If you don't have the paper copy that came with the tub, your tub manufacturer should have the installation instructions posted on its website. I'd at least research the clips before proceeding to the tiling. When the tub is filled with water, or a bather steps in to the tub to shower, the weight and movement of the bather could cause deflection in the tub. Excessive tub or flange movement/flexing could result in failed caulk at the tile-tub joint, or with more movement, grout and tile issues on the bottom courses of wall tile. Good luck, I hope you can get this sorted out.

You are essentially building a new shower complete with new tiled walls. In my experience the cost would be nominal because the pipes are all exposed. When I did a gut remodel they did all new pipes anyway - I mean in the scheme of things the walls are open and pipes are cheap versus the cost of old pipes needing repair down the line I had them run pipes for an additional hand held shower head at one end of my shower by the bench. No issues and didn't seem to raise the price much. Again they did it at the stage when everything was exposed and they were doing new pipes anyway