I'm planning to do a hike from Gubeikou to Jinshanling.

I suppose that this is fully doable and could be done with the usual described route with the detour around the military zone etc.

Previously I had imagined of going even further to Simatai, but it looks like that's not an option as there are gates and guards that don't let you to do this. Is it so?

Could someone please give some additional info about the 2nd travel option? Where and how could the bus be picked up from Jin shan ling etc. After 16:00 the only option to get back to Beijing would be taking a taxi to Miyun for ~100 CNY and then taking the 980 bus?

Is there anything else I should consider regarding a trip to Gubeikou and hike to Jin shan ling?