As a Croatian, living in Croatia, I can re-affirm your position. He would get extradited with a complimentary cake for CIA in no time at all. Our ties with US are deeper than the ones with EU, and we are a EU member. Hell, even our chief intelligence officer has dual citizenship (US and Croatian).
Not to mention it isn't like the US and Croatia haven't worked together at least once in apprehending a criminal living in Croatia wanted by the United States.
American and Croatian police have worked together to arrest Ukrainian hacker Sergei Litvinenko in Croatia, reports Croatian radiotelevision.
Litvinenko was wanted by US federal police for fraud he had committed in the United States. The federal courts had issued a warrant for his arrest and he was caught in Croatia earlier this week.
There is literally no reason why they wouldn't extradite such a highly valued criminal to the US, formal treaty or not. The ties between the two countries are too strong.
