Is it possible to drive by car from Jakoruda to Samokov via Granicar? I have such a road on the roadmap of Bulgaria.
No, it isn't possible.
Is it possible (and allowed) to drive farther than to Rilski Monastir? I mean in the direction Ribni Ezera. If so, how far?
It is possible to drive several kilometers after the Rilski Monastir to the place "Kirilova Poljana". This is the endpoint of the asphalt road.

Колко малко хора знаят
за тоя чуден горе кът
и колко много долу пъплят,
невиждащи нагоре път.