I've been adding them to pretty much all my big plumbing jobs. Kitchen sink was leaking... Added shut off. My water heater now has 2 shutoffs within 2 feet of easier... Though that's more so I can flush it yearly as an on demand heater. And now my shower has shut offs. The only plumbing I haven't touched yet is my half bath. Not touching it until I plan to renovate.

I did this when I broke the cartridge trying to replace the handle.

Also took the chance to add shut off valves right before it hits the shower so any changes in the future should be way easier

You can go with option 1, but glue the entire board to the frame. Let that set over night before sleeping on it again.

You can also try drilling through the frame and board and using something like a flat head bolt on the outside of the frame and tighten it down. Id still use glue though.