If it were a server merge, they would have done it before resetting the ladder.
They would have to rewrite a lot of stuff, but they could easily solve duplicate character names by just making duplicates say Name@USEast and Name@Europe on a merged realm, but I'll be honest, I don't know how they would do the same for account names if there are duplicates, because account names are already formatted like account@useast.battle.net
Seems like something it would be cheaper to just run the cheap game servers instead of investing dev resources, which we know blizzard hates doing. So, I don't see a realm merge happening.