When the machine heats up, is there a noticeable increase in fan speed and the volume of hot air expelled from the vents? If not, it could be the fan bearings wearing and preventing it from spinning up fast enough. Might need a new fan unit to fix.
Since the machine has been running hot, Bob's suggestion of dried out thermal paste is very possible.
Looking at some of the videos, it looks to have some substantial heat pipes from both the CPU and GPU to the radiator. I've never seen them fail but I suppose it's just possible something is blocking them.
Also looking at the videos, there is some significant careful leverage necessary to get the base cover off and obviously a chance of breaking it. And getting the motherboard out has a significant possibility of doing some serious damage internally. Although I do some hardware maintenance on my own kit, I think I would be inclined to take this one to a service agent, even to replace the thermal paste and or fan assembly.