OMFG this site sometimes... Wrote a response, timed out, everything gone.
In short:
Have you tried using another trench wall? I am fairly certain there are vaultable ones and non-vaultable ones. Same with sandbags.
From what I remember from playing around with hulldowns, objects are spawned in this way in relation to the entity's facing. So if there's a squad of infantry, I think there's ample opportunity for some facing to get a little screwy as those entities maneuver around.
The standard trench wall segments don't work for the rear wall. It's not possible to change the direction of a spawned object so you end up with a trench like the second image, with one wall facing the wrong way. I've made a new version that solves some of those problems.
The rear wall cannot be vaulted over. But the front wall can.
I want to make them removable with wire cutters, although it doesn't seem to want to work. They are currently unremovable by any means.
They Block line of sight now, as demonstrated by this conscript squad. The rear echelon cannot see the conscripts, nor can the conscripts see the rear echelons. Great for ambushes!
The trenches do not however, block rifle rounds. The conscripts were able to quickly shred the rear echelons when fog of war was turned off despite being prone.
Cover is inconstant within the trench. When running down it's length, the squad will repeatedly be in and out of cover. The whole cover system is utterly bewildering and I have no idea how to make the entire trench a directional cover zone. Making it omnidirectional would be easy, but undesirable.