This probably doesn’t have anything to do with your screen. It’s your internal hardware. If it is still a fully original phone, you need to realize it’s +5 years old. Parts wear and can break down after a long while. Make sure your phone’s storage is not completely full, as this can slow down your phone. If this doesn’t work, try software-update (you actually need to do these always, anyways :D). If it still doesn’t work, make a back-up, reset the phone, and put the back-up back on there.

Maybe there is a problem with your internal power supply (battery & wiring). Check if your battery connector is still properly in place. If yes, you maybe have a damaged or old battery. It could be that your battery reached the end of it’s life, and it’s not able to deliver enough power to get the screen to work.

First of all, a cpu temperature above 100 degrees is not normal! I think a heat sensor is broken, because you mention that your cpu is fine when the fans go on max speed. The computer is supposed to run those fans at max speed automatically if the cpu get’s a high load (what increases the temperature). Another thing that could be a problem too is that your airflow isn’t that good anymore. It maybe doesn’t seem like it, but maybe one of your fans (or fan motor) is damaged?

You cannot remove the back panel of this device. The motherboard & battery etc. are stuck on the back panel. The front (glass) panel is removable. There is a guide on the site on how to do that.