Bringing your posts together...
Whale cockpit shower
Ok, I see you took off the shiny
Knobs to the HOT and COLD
Valves.
The
Valve is that screw head.
If you are worried about unscrewing it...
1) put a tray under the
Valve to catch parts...
2) unscrew it slowly and observe the stem components.
But...
You should
not have to unscrew it completely. Just enough to allow
Valve gasket seal to release.
How do I know this?
I had the same problem and drove me crazy until found the damn
Valve stem sticking the gasket.

______
Tip: The
Knob may turn but the stuck
Valve stem doesn't and it is the
Valve , not the
Knob that adjusts flow.
Jim...