Two strings, isn't it? It's very much like an erhu, and also some middle eastern instruments (rababa?). Similar sort of idea and construction.
That depends. Gusle contains one string (although a Serbian variant has two) I believe. The kind I have seen has one string, made of horsehair and bow string made of horsetail...it's odd, middle-eastern sounding but in the Balkans, influenced by Ottoman culture for sure.