I had a very simaler problem in operation flashpoint. I solved i by putting myself as the enemy squad leader, and walking down with them, and then seeing where they got stuck and adding somthing like a jeep wreak or obstacle a meter or 2 before the spot, so they wont head that way. but they still got stuck sometimes.
good luck with your probem.
EDIT:
I just set up your senario, I get the same thing, and how they all stop at a certain point makes me think it has somthing to do with the AI paths. because I set them up elseware and they can go down stuff allot steeper than that.
even stranger is when I put them at the bottom and tell them to go up, they all start swiming out to sea.
I have a feeling this is not a problem you can fix with waypoint changes or domove.