I don't see why not.

Usually when you say "wet room" in includes a bathtub. But you mean just a walk-in shower with no curb, correct? If so, then sure, that could work. I'd also think about putting in a pocket door to save space from the door swing.

Yes that can work. You want THE BEST tiler in your area and make sure he wraps the waterproofing up the wall around the room. Yes to pocket door and if you can stretch the budget a wall hung toilet would buy you more space as well.

Maybe? Might depend on how much time and $$$ you got for it. The floor venting is an issue. So is that window. It's currently outside the wet area. And appears to have a wooden frame. You would need to make that exterior level waterproof with the window replacement to account for it being in the wet area if you did a wet room. The whole floor would need to be pitched for correct draining, and that would depend on what your flooring/subflooring has going on. Especially since that room is small and you have concerns about floods you will need to be extra careful with that pitch. Whatever is currently being contained by that curb and shower door will spread to much more of the room.

In a wet room, the entire floor needs to slope to the drain at a rate of minimum 1/4" per foot or maximum 1/2" per foot. You would need to think how you would accomplish that slope within your existing flooring structure which is flat. A shower curb and/or shower pan allows the 2" above the drain to be built into just the shower area, which you won't be able to do if you do a full wet room.

It would certainly be feasible to waterproof the floor under tile using something like one of the Schluter systems regardless if you make the whole room wet room with full slope to the drain. This would help protect against water damage, though would probably still require cleanup to address standing water.

One concern I would have is that if you have elderly people going into a tiled bathroom where water is not well contained into the shower area, you're creating additional slip and fall hazard. Someone takes a shower and then an elderly person comes in to use the toilet and may slip and fall. Even if the entire floor slopes so the water isn't pooling, wet tile itself can still be somewhat slippery even if it's small format and/or high friction.

Thanks all! Yes, just want a curbless shower. Sadly can't do a pocket door. There is only 12" of wall on one side of the doorway, 6" of wall on the other.

Pricklypear, thanks for the pointer on a slippery floor. I'll have to think about that. If we're stuck with using a shower pan, adding Schluter waterproofing underneath the rest of the floor could buy some peace of mind. I still plan to mop up the splashes. Just would like a little insurance, so I don't have to stand watch for her entire bathroom time.

If you want to spend 30K for waterproofing and tiling a giant shower, then maybe. The structural alterations needed to get the slope for the whole room are only going to be able to be determined if possible by someone on site. A conventional foundation with joists is the most difficult and the most expensive of the possibilities. If it is a concrete slab foundation, cutting out concrete is filthy but not complicated. Unless it is a post tensioned slab. Then it is not possible without X rays and super careful demo, with more X-rays throughout the process. Might as well forget it entirely if it is a post tensioned slab.

All of the things mentioned this is a complicated job and open to many issues. IMO with people using it that have disabilities an out swing door is a must BTW since no possibility of a pocket door. Is you daugeter in wheelchair or soon to be ? You need a tile setter who has done wet rooms this is not a job for your friend who does tile . I think if a wheelchair is something needed to be planned for this whole space is too small for that . IMO a low curb if at all pssible just a much better idea. in this small space . BTW a to scale plan is a much more helpful drawing . Easy to just make each square on graph paper 6" . Post the new drawing here in a comment all things related to this dilemma now dealt with here in comments .