"Also, a lot of seriously dug in positions have very limited ways of getting in and out, and are meant to be static positions (see revetments for AFVs in firebases in Vietnam), so being able to leave by moving in any which way is similarly inaccurate."
Frankly I had in mind the more temporary dug in positions
meant for a prepared defense. I would guess you are right that there might be cases where getting on the move again isn't an immediate concern.
"On the matter, what kind of protection is actually afforded when you entrench and get the little sandbag wall icon around the tank? Does it get HEAT armor? Extra armor? Reduced critical hit chance? Is that in the manual (I'm at work, so I can't check ATM)?"
As I recall only the turret can be hit. Since the turret is usually better armored than the hull that is quite an
advantage.
Perhaps there might be a lower chance to hit the tank to represent the smaller target and/or spotting bonus or whatever but I don't know for sure about that.
"I have never seen it in the manuel but I think they get great bonus to steel armor(just looked in iraq oob and a t55 has a armor value of f=19 s=8 b=6, with it being in the cement it has a value of f=59 s=48 b=45.)"
I suspect that the original OOB designer meant for them to be heavily dug in tanks. Most bunkers in the game have unrealistic values anyway but with 92 nations in the game to deal with I doubt the design team will have time to look at it before the 2040s...
Speaking of iraqi bunkers, a few examples:
http://flickr.com/photos/75031815@N00/250765267/
http://news.webshots.com/photo/10968...16222121LCIdAw
http://www.flickr.com/photos/milans/28528251/
The great majority were however simple dug shelter like this:
http://www.flickr.com/photos/12bravo...7594073501593/