Replace shower Plumbing Valve assembly shower head 2 knobs 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???

The other side of the fiberglass shower corner shell is a bedroom wall. Yes if a pipe snaps while working in the wall through the fiberglass shell side inside the shower I could get at it
from the bedroom wall.

Could I use a slip cap to shut off the water from through the bedroom wall? So I wouldn't have to have the water to the building turned off while I do this repair/replace?

Is there standard shower plumbing assembly's?
Or is each booger built custom to the unit?