How to Remove a stripped Symmons shower valve seat, Symmons Shower Valve Repair

I was thinking the worst when this happened. I had purchased the kit months ago but had procrastinated the repair. This is a big house with several people and bathrooms. When I ruined the old seat trying to remove it and had water gushing out of the faucet I feared a huge job. I envisioned removing siding and sheeting from the exterior of the house and replacing the entire valve assembly.