In a fiberglass corner shower shell.
What I have is an old leaking rusted
Shower valve assembly. It leaks from the
Shower head that is a hand sprayer with a wall mount from the sprayer. It also leaks around the both the hot and cold water wall mount knobs. The cold water knob the sleeve that is against the wall hangs loose. Won't stay put.

So I need to replace the shower faucet/knobs/valves behind the fiberglass shell without damaging the fiberglass hopefully?

Can I buy the assembly complete?
It is called a??

What is my best plan of attack to get the shower valve assembly out of the fiberglass wall?

I need to shut the water off for the building because it being a shower it has no shut off valve like you have for a bathroom sink or toilet. Unless I can turn off the water to that unit by??

"What is my best plan of attack to get the shower valve assembly out of the fiberglass wall? "

Open up the wall behind the shower enclosure.

"Can I buy the assembly complete?"

No, each shower is a little different from valve to drop eared ell, so the shower raiser is always added. Good luck, Tom

So just go in from the bedroom wall from the gate is the best way to get this done right then? Don't even mess with going in on the fiberglass /inside the shower side until I am buttoning up the shower head and knobs.

How can I cap off the water in the shower wall so I can work and not have the water to the whole property turned off while I re-construct the assembly needed for the repair? With a hardware/plumbing supply store stop in the middle. I don't know the correct term, with slip valves/caps?

So just go in from the bedroom wall from the gate is the best way to get this done right then? Don't even mess with going in on the fiberglass /inside the shower side until I am buttoning up the shower head and knobs.

Yes!

How can I cap off the water in the shower wall so I can work and not have the water to the whole property turned off while I re-construct the assembly needed for the repair?

Shut the water off to the house and install ball inline valves on the supplies next to the floor. When you drywall in the back wall leave a access hole to get into the shut off valves if needed.
Good luck, Tom