what about buses? Any regular bus connection instead?
Well, maybe Maxy can correct us, but we didn't find any regular cross-border bus lines in that region. Even buses like Przemysl-Stryj go not via Nyzhankovychi-Khyriv, but via more common Medyka-Shegyni, so it's actually might be a difficult task to travel by public transport from Sanok to Khyriv via Kroscienko-Smilnytsia border crossing
Among buses from bus stations - I'm sure only about bus Truskavets - Plzen - it uses border-crossing in Smilnytsia-Kroscienko, but this bus doesn't have tariff stops in Poland - so minimal possible travel on this bus is Smilnytsia - Hradec Kralove - from last Ukrainian stop to first stop in Czech Republic :-(
Ustrzyky Dolne is known destination among Leopolitans for skiing, so some tourist agencies in Lviv provide "charter" buses Lviv - Ustrzyky Dolne. Buying only bus transfer (without hotel in Ustrzyky and other tourist services) is usually possible, paying only for OW transfer - too, but the problem is to find the agency that arrange a transfer in the day you need and deal with them. It can be not an easy for foreigner.