I had another go at getting that chrome cover off.

It was silicone'd in place :)

Looks like my mixing valve is connected with PEX for the Hot, Cold and up to the shower head.

The connection down to the tub spout is soldered copper.

The other side of the wall is in a laundry room and the wall area is clear.

I did find a video that showed the procedure for cutting the wall and replacing the mixing valve.

I am not sure I want to tackle replacing the valve at this time.

That's a good project for when I have a whole day available.

I would have to borrow a PEX tool.

I wonder if the threaded on connectors could be spun without cutting the PEX?

Would that destroy the PEX seal?

I have all the pipe soldering tools and copper pipe fittings.

Here's a couple pics of the round brass valve/mixer and the valve itself.

You can see the PEX connections left, right and up