I just did this hike with a private guide and driver a couple of weeks ago. Forgive me that I don't know the name of any of the roads or towns...
We drove up from Beijing and pulled over in a large car park just past the
ruins of a few watchtowers on the left side of the road. From there, we walked back (south?) for a minute before turning left on a small
driveway. After passing through a cluster of dwellings, we veered off to the right onto a
trail and started climbing. My guide (as best as I can tell) took the wrong path and we ended going back downhill. No problem... he just led me through the brush straight up the mountain until we reached
the Wall.
As far as the military reservation, it was pretty easy to figure out where to get off the Wall. The entrance to one of the watchtowers was sealed off with plywood and we had to back-track to the last tower and follow a parallel trail on the left side of the Wall.
I don't know if we made a wrong turn or what, but we descended down into a valley and ran into some local folks that were not happy to see us. My guide had to pay a trespass fee to the first one and then argue with the next two. After regaining the Wall, it was a short distance to Jinshanling.
Hope that helps.