Looking at the drawing, the head of the plug will bottom on the head surface which stops applying any more pressure on the o-ring. So over torquing shouldn't damage the o-ring. The desired o-ring squeeze is designed into the port depth and diameters based on the head bottoming. Torque won't dictate squeeze.

Also in my opinion , the main reasons for o-ring failure is the way they were installed or the wrong o-ring was used based on temperature. We teach our employees to lube, finesse them into position and not just crank them in. A clockwise counterclockwise ( back in forth motion) is helpful. Like, In 1/2 turn, Out 1/4 turn, In 1/2 turn.etc. It helps the o-ring to seat properly without bunching and causing pinching while the o-ring compresses. If the ports, o-ring size, material type, temperature rating and the installation was correct, the o-rings can take the pressure almost forever. Temperature could still take the "forever" away and the o-ring could still break down and fail.

Edit: Loctite isn't necessary .. The problem with it, if you use loctite and remove the fitting, The loctite residue that is on the threads can fall inside. Ever pull a bolt out that had dried loctite on the threads? Will it hurt with that stuff floating around inside the heads?.

Buna only rated to 250 degrees.
Viton to 400.
Epdm. Looks actually like buna. Not for petroleum usage. Slightly purple with the right lighting.
Nice little write up.
O-Ring Material Selection Guidelines, Information
Viton-> Auto brake fluids? Did you know? O-Ring Material Guide

 

Yes loctite drys up and some can fall into the oil journals if pulled out. But if the fittings aren't leaking because you used loctite why are you pulling them out?

 

Most folks use them to drain oil for any kind of high pressure oil system work like changing injectors..................... or when the loctite fails on the injector cups.

 

How do the fittings and plugs on the top of the head drain the oil? From what I read most people use the rear injectors to drain the fluids in the heads for injectors cups and failed injector o-rings.