Helicopter autorotative descent: would need to take a peak at the Hip's flight manual, but I'd guess 3-5000 FPM is in the ball park.
As to "height above terrain" this story makes me scratch my head a bit. Helicopter up at 12K is an altitude that must have been chosen for a purpose. With Damascus roughly 2200 feet above MSL, that's still about 10K AGL ... which leads me to guess that this flight profile was chosen to avoid small arms fire. (I may be wrong.)
But it got hit anyway? Seems to me that some actual AAA like a 57mm might be involved if that were the case.
Ran into another plane because neither cockpit crew saw one another? Most likely explanation from where I sit.
A lot I don't "get" about this one.