FIrst, it's rare to have shutoffs for a shower valve in the wall. Yes, they are there sometimes, but since you don't need to take the eschution off to change a cartridge, most people wouldn't see them, even if they were there. Now, if there's an access panel on the back side, it would make a bit more sense. So, yes, you have to shut the whole house's water off (unless you have something like a manifold home run system supplying the water to fixtures). Can't help you with the proper cartridge, and a pro may need to see it with the handle off, so yes, it does look like two trips for you. Hopefully, the thing comes out easily...some of them, depending on how long its been there and how hard your water is, can be a real pain to remove. Yours may just pop out easily, you may need one of the removal tools, and the skill to use it!