Plumber has advised it's difficult to remove bonnet and cartridge without breaking connection to valve on old style Valley single handle shower faucet. But because the "antique brass" trim is so expensive I don't want to have to purchase and possibly ship back standby faucet just in case. So is there any interim fix so the house water can be turned back on if valve breaks? Wouldn't need to use shower.
Second part of this is plumber suggested I lubricate faucet in advance of visit. We have hard water. What is best method to loosen this without discoloring the wall tile and/or getting liquid in the wall cavity. Do I have to remove the handle with allen wrench and pull out the big round flange to do this effectively?
How difficult is it to install an access panel in the drywall after this is over?
Would really appreciate other ideas and advice.

earlyboomer