Thanks for the feedback again,
Following ngtnewington Newington's reasoning, I think installing the check valve could solve the consequence, but not the cause of the problem. I'll probably do it if nothing else works.
Changing the tap as Thomas Peacock suggested, I will no doubt have a better system, but I haven't solved the source of the problem either.
I did some tests yesterday, removing the core from the galley faucet, and cleaning up the calcium buildup in the water mixing chamber, where through the oring could be running hot water from side to side. I reassembled the system with silicone grease, and the same problem happened again.
I considered swapping the core of the bathroom sink faucets for Galey's and testing it again.
I need to figure out how to lower the boiler water temperature, because I live in the marina and leave very little to navigate using the engine.
Good puzzle I got for myself.
Jose Prieto SY Wayag SM323