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If you are going to open up everything and shut off the water to replace the valve, the best solution is to put the valve somewhere else. Either up high on the wall so it won’t get wet or turn it to access it from the other side of the wall cavity.
If I move it higher, it will take on less water. But there would still be some water unless I move it all the way to the ceiling, which would really lengthen the path the water must go.
Regarding the other idea, it seems the house was extended at one point in time. So the wall was once an exterior wall and is very thick (~50cm?) and solid (concrete, brick, or combo of the two). Not sure if I’d be weakening it structurally to do that. It’s in a corner though, so there is perhaps an interior wall I could move it to the other side of. But I think it’s brick & thus also structural. Perhaps feasible though. It’s hard to be enthusiastic about demolishing part of a plaster & lath bedroom wall to reach it, but it could be the best option. I have to wonder if the original builders had a good reason for not doing that. Tricky though since I’ve seen plenty of dodgy work from the past builders of this house.
There is another shower underneath with easily removable PVC ceiling and access to the pipe. So indeed theoretically I could put the valve in the shower ceiling of the lower apartment. But that segment of pipe is galvanized steel, so I would have to cut it and add threads. I actually have the ratcheting tool to do that but I would still need to buy the threading die. It’d be a bit weird if the lower apartment had the control over the water above. Not a show stopper but I think there are better options.