Hi Wojtek,

You better be prepared to walk alone but please keep in touch. You may be lucky and find a partner. The trip would be rather easier and more informative for you.

Pirin: yes, you can get to Sandanski hut by car. I think it is all the same traversing the Northern ridge from south to north ot back. However, from north you reach the ridge faster and it seems that there are more options for getting back by another route.

Rila: what you write is correct.

Stara planina: Your estimate for the length of Karlowo - Ravnec - Rajsko Praskalo - Botev - Goljam Kupen – Karlovo is correct. Of course it depends on the person who does it. Anyway, it is long and strenuous; about 2500 m are to be climbed.
You can see some parts of the Dzhendems (they are two – North & South Dzhendem) from above but you will not see much, just some general views. Those are rather precipitous places where almost no trails exist; trying to get there "on the fly" is not advisable.
There are little busses between Srednogorie (btw, that town no longer exists, it has been divided to reestablish the former Zlatitsa and Pirdop) and Klisura but I do not know how often they travel.
The only paved pass between Tetevenska and Kaloferska Planina is Troyan pass (between Troyan and Karnare).

Good luck!