The great majority of Muslims preferred to identify as undecided or as Yugoslav. Also considering how quickly they dropped those other identifications once the Muslim nationality became available, it doesn't seem as if many Muslims really believed they were Serbs or Croats.
I'm sorry, but I don't really believe this idea of a mass immigration of Albanians in Kosovo. Totalitarian states generally don't permit their citizens to leave and Albania was one of the most totalitarian ones and had plenty of ways to stop emigrants. And while Yugoslavia may have been one of the most liberal communist states, seems rather unlikely that illegal immigrants would be able to enter a society where you are basically lost without legal documents.
Also, a detailed look on the demographics do not seem to bear this out. Kosovo's Albanian population grew by about 220% between 1948 and 1991, which is higher than Albania's 183%, but one must consider that Albania was quite poorer and didn't have only Albanians, anyway.
You can choose not to believe it, but most the books i've read that talk about the history of tito's yugoslavia mention illegal albanian immigration into Kosovo as a huge problem. It is true that Totalitarian states don't want their citizens to leave their states, but citizens definetly do want to leave totalitarian states. And Albanian immigration into Kosovo wasn't the result of the Albanian regime not allowing its citizen to leave, but it was the result of the Kosovo authorities not policing the border between Albania and Yugoslavia and allowing Albanians to enter, especially after the fall of Rankovic in 1968 and the passing of the 1974 constitution which gave Albanians in Kosovo all de-facto authority in the province.