Yes, there are temporary grab bars. The wall ones probably would be dangerous in your situation, though, since you don't have a really solid surface. There are grab bars that mount to the tub side, like these. (not endorsing this site; it's just the first one that came up when I searched).

Grout isn't waterproof anyway. Moisture can and does get through both tile and grout. Your shower is supposed to be waterproofed before the tile goes up.

There are floor to ceiling grab bars, but the are not inexpensive.

Here is a link that might be useful: pole grab bars

Silicone. It also works as a low to medium strength adhesive, which should help hold the tiles together.

Or you could use tuck tape, it's very strong and quite waterproof.

Thanks! now that I know what I am looking for, I see the grab bars and poles on Amazon. Excellent - I might actually get one by Thanksgiving!

Silicone sealant - I will look for it. Thanks!

I'd rent an accessible hotel room nearby for your MIL instead of you trying to spend money on something temporary and worrying over the whole situation. They will have the roll in shower with the grab bars, and it won't cost you very much over the temp measures you are thinking of for here. Some hotels even have accessible adjoining rooms if she travels with a caregiver.

Believe me, if I could get her to a hotel I would. In Laws are not known for compliance with rational thought.

Meanwhile, I got some DAP quikseal plus with silicone, to patch the worst of the grout, and some rustoleum Leakseal which seems to be a flexible rubber coating in a spray can. That should hold the tile for as long as she is here. Plus we got a grab rail. So we should be okay, at least for the bathroom part. Thanks!