I would do all the tests possible to check out the head gaskets integrity. It is now the most probable suspect.

Since you have been losing some coolant right along one of the injectors tips might look cleaner and the pre chamber as well to some extent. Adding coolant to that cylinder with a leak may have steam cleaned everything in that cylinder over the last while before it became serious.

What to me proves the hydrolocking is you got almost two turns on the engine before it stopped up. On one rotation of the crank the valves would be open. The next rotation closed and the fluid would be trapped.

It was just luck where you started turning. Or by turning to the stop point each way you where going by the open valves each time.

I still always like to identify the cylinder impacted with any problem before removing the head. In your case the coolant will have left evidence. If your fault had been actually mechanical it would not have cured itself no matter how long it sat.