HI, How can I get from Simatai to Tianyun Mountain Glass passage and then go back to Beijing?

Well, public transport is quite inconvenient. You are advised to hire a car for the whole trip. It may cost around CNY 800 per day. :)

A hire car is unfortunately not an option as I will walk from Jinshanling to Simatia. No other options? Should I delete it from my program?

Oh, I see. You can take bus line Mi 55 from Simatai to Miyun Bus Station. Next, take bus line Jiao 82 to Pinggu Bus Station. Then, take bus line Ping 25 to the terminus. Afterwards, take a taxi to Tianyun Mountain for about CNY 5.

If you don't want to change buses, you may consider deleting the mountain. :)

And ... What will be the price per Taxi from Simatai to Tianyun Mountain Glass (return) ?

The fare may be around CNY 250.