Overheating: thermal paste or thermal pad a possible fix?

The LG G3 SOC gets really hot and then the thermal throttle kicks in, and the 2.5 Ghz becomes 1.7.

I've been studying this for some time and it appears that it's very simple to add some thermal paste or pad to thermally connect the SOC to the internal metal chassis.

Some people have done it and the G3 drops about 9C, and the thermal throttle almost disappear.

I have 3 questions about it:

1) Why LG did not put the thermal pad?

2) Thermal paste or thermal pad?

3) The SOC will be heating the chassis that is right behind the LCD screen. Will the heat damage the screen?