The compression and the exhaust will have nothing to do with the overheating.
The coolant system is actually a pretty simple system.
Water flows from the radiator, thru the pump....
The pump pushes water into the block.
The block heats the water up and forces the thermastat open...
The water flows back into the radiator, cools and starts the process all over again.
That is all there is to it....literally.
Anything that will cause overheating will be found in this path.
My first thought was that the thermatat was sticking but you said that you changed that.
My second thought was that the water pump is going out.
Might be an issue with the impeller or the seal in the pump. (Why you are losing fluid.)
If I were a betting man though, I would say that the radiator is partially clogged.
Flushing out the system really good might help.