I will quote some of what you wrote:
"had been motor sailing for around 2 hours at 2200 rpm in light winds. Without warning the motor pitch changed and the engine dropped around 400 rpm and the temp rose immediately to 205 when the "Hot Engine" alarm came on.This all occurred within less than 5 seconds. We had just at the same time been hit by a very small wave on our starboard side-coincidence?"
"Coolant level-reservoir-EMPTY. Previously FULL in the morning."
"I added more fluid (approx 2-3 quarts in total!)"
"Again today the reservoir was empty at the end of the trip and the engine took around 3 quarts again to fill. The mechanic felt the water was "escaping" from the engine somehow."
"At present, the engine will not start at all. It does turn over, however, will not fire. The battery is 12.7 V, fuel 40% visual and on gauge as well."
While I still wouldnt rule out a head gasket, upon further thought I would be looking at the heat exchanger as the primary cause, with the overheat happening after heavy seas to starboard. The engine cooling system is sealed, or should be, and no external pressures should have any effect on it. The only area where the system could have contact to outside pressure is at the heat exchanger.
The fuel system issues are likely coincidental and unrelated. Its one of the problems we run into with engine diagnosis involving engines that lacked regular maintenance or are new to us, you can have multiple issues with multiple operating defects that point toward things that aren't even related.
I would pull the exchanger off and have it pressure tested, just to rule it out. Then go through the cooling and fuel systems thoroughly. I dont know whats available in the DR, but here in the states the auto parts stores sell test kits to test for combustion products in the cooling system. I would test for it if you can find a kit. I would also remove and visually inspect your coolant hoses. On two occasions in over 40 years of wrenching, ive ran across embolisms in a coolant hose. That can be a real nightmare to diagnose, let me tell ya.