I think it is clear that he is Serbian, I just wonder in which conflict he fought, at the encounter with Darko Brevic, Darko states that he betrayed their unit, because their unit killed friends/neighbours of him.
Maybe they were Bosnian Serbs in Bosnia where Bosnians and Serbs lived side by side for a long time and Darko felt more affiliated with his Bosnian neigbours, I think this is more likely than he felt affiliated with Albanians, because Serbs have been a long time a minority in Kosovo and lived rather seperated or I am writing nonsense?
That Roman marries in a Catholic Church is no clue at all, he is most likely Atheist/Agnostic.
BTW I only recently met a guy who is Serb, speaks Croatian, is a Muslim and looks like an Arab. At the end we do not definitly know on which side Niko fought in which war, though he hardly is a Muslim. Usually Russians and Orthodox Serbs are said to get along well, this might be a hint, that Niko is something else. Neither Bellic/Bellič, Brevic nor Cravic seem to be actual names, this means Niko might have a totally fictitious background.
PS: I think it is most likely that Niko is an Ethnic Serb from Croatia and that he was involved in the fightings in the
Republic of Serbian Krajina and that his mother still was living in Croatia when the game takes place (Croatia is apart from Slowenia the only reasonable place where rich foreigners would buy property as mentioned in E-Mails but Slowenia saw little violence during the wars). Croatia had a Serbian minority even after the wars. If understand right, Niko's unit commited crimes against locals which was the reason for Darkos betrayal (dialogue at that their encounter) but also Niko's faction suffered atrocities (dialogue with Faustins wife), both Serbs and Croatians commited war crimes in the region.