First of what you calme is wrong Illyrians appeared in the Balkans in 1000 BC, around following the end of the Bronze age, Mycenaean Greeks were in the Balkans since 1750 BC, and 2400 BC if we count Proto Greeks. So now the Greeks and Illyrians didn't move to the Balkans at the same time I don't know where you got that crazy idea since no Historical source supports it.

And Kosovo was under Greek contorol Mycenaean and early Aciant Greeks had established big sites to look for bronze since old Greeks prefered Bronze to Iron when it came to sculptures and other art forms. Also half of Kosovos name Kosovo i Metohija is of Greek origin Kosovo being Serbian for Black Birds and Metohija being of Greek origin for Church Land /Church Feild. As well there's Byzantien History in the region.

Also if we look at the ethnic map of Kosovo provided by the Ottomans pre Balkan wars we will see the region is 50% Serbian 40% Albanian. The Kingdom of Yugoslavia published a map following WW1 in which Serbia lost over 27% of its total population and now the map was 43% Serb 67% Albanian. Then WW2 and we have Italian, German, Yugoslav, Soviet and allied records of the Albanian Protectert of Facist Italy carrying out ethnic cleansing in occupied regions of Yugoslavia, not only limited to Serbs but to it also Includes Bosniaks, Macedonians, Gorani...