[MarkSheppard]

Which way is front?

I assume you mean the side pointing towards the enemy? It's the one with the cloche on it. (right)

If the right side, then is this designed as an shelter against artillery?

Pretty much yeah. It's actually a two level bunker, with room to sleep 50~ people.

If to the left, is the grey section ("2" on the drawing) a fighting trench?

No, it's a fougasse. Modern day version of the ancient moat.

The two (1) items in the diagram are actually drawbridges which can be cranked in or pulled up, and as a bonus, if you try to throw a grenade or satchel charge at the entrance, the charge will end up inside the fougasse and explode away from the entrance.

It seems a bit wasteful to provide 2.25m of concrete for a hidey hole with only a couple of mgs.

2.25m is actually quite light, being only Level 2 and protects from 240mm weapons. The French went up to level four and protection from 400~mm weapons in their bunkers.

As for the few MGs, yeah, it does seem wasteful, but it's primary purpose is to provide a gastight, artillery proof barracks for 50~ men (possibly as high as 100 if hotbunking is used), which is connected via an underground tunnel to other fighting bunkers and positions.