Does the joint where the glass wall tile meets the deck need a special glass molding? I would prefer none just glass tile to solid surface. You can run the wall tile right to the tub deck, and caulk the intersection with color and texture matched caulk. Borders/liners are added for aesthetic and transitional interest. Sometimes it's purely design, sometimes for function, sometimes a combination. If your glass tile can stand on its own, and you'd prefer it to do so, then let it. I've not ever seen a glass grab bar or hand rail. Clear polycarbonate (I think it was PC) yes, but glass, no. Mongo

Ameriwall has a tub surround with a 72" back wall. It can be cut down for a smaller tub. Might be others, that's the first one I found. 7230TA

Since you have 3 bathrooms that sound quite large, I would leave a tub in one of them and then do exactly what you want in the other 2 bathrooms. Whoever designed your house sure did love the long vanities! I'm stunned by those measurements. It could look a little odd to have counters that long with just one sink, so I would consider adding tall storage cabinets (if you could use more storage) alongside a shorter length countertop/vanity. As for one or two sinks, I'm building our new main bath with just one, because I like to have counter space to spread out on. That's fine for the way we live - my husband and I never need to be in the bathroom getting ready at the same time. Nor did my kids ever have a problem sharing a single sink in our previous house. But some people would refuse to buy a house that had just one sink in the master bath. I don't think there's any one right answer for this.

Tub surrounds are for tub/shower combos. Generally on a soaking tub, tile is done, but you can get Swanstone or Corian walls to any dimension. If you are planning a tub/shower for this setup, be sure it has the integral tile flange as that's needed to keep water from penetrating into the walls.