Not a bad idea, I never thought of that. I know, in at least some places (like mine), risers going up from the basement (at least to the second story) have to have shutoffs. I don’t know the details of it, but I have an acquaintance that’s a plumbing inspector. I was asking him a plumbing question one day and mentioned something about not wanting to shut off the water to the house to do something in my parents second story bathroom. Without even thinking, he said that, by code, there’s a shut off in the basement for the bathroom. He was right.
In any case, my point was that if the OP has a multi story house and this is a second floor, there may be a way to isolate the plumbing to the bathroom and keep the water on for the rest of the house.
I’m hoping he gets it taken care of on his own. A lot of plumbing work is easy, but overwhelming if you’ve never done it and I probably would have suggested a handyman from the beginning if I had thought far enough out to realize that once he removes the valve under the toilet, there’s no going back and he’s got no water until it’s fixed (unless there’s another shutoff upstream). That’s why I shifted to pushing for him to use a sharkbite.