No other parts are accessible without opening the wall up.

No. No opportunity for it to freeze. Interior wall runs and it would have thawed and or burst by now with outside temps back above freezing.

It is a Kohler single handle. Nothing changes regardless of replacing. My only guess is something clogged in the hot feeder pipe or in the copper/brass valve body.

I have added pics of the parts I replaced. These are the only accessible parts without cutting into the wall.

Can’t see it without opening the wall up. Thx.

I think that is what I essentially replaced. I took the handle off and replaced all the parts I could access behind the handle. The only other replaceable part is the shower head itself.