If it had chosen to go further, it could also have explained why Kosovo was never legally part of contemporary Serbia until 1945: the treaties of London (1913) and Istanbul (1914), by which the Ottoman empire ceded it to the Kingdom of Serbia, were never ratified.
The legal status of Kosovo in the Kingdom of the Serbs, the Croats and Slovenes (SHS), called "Yugoslavia" after 1929, is even also in question. The Treaty of Sèvres, signed by the SHS Kingdom with Turkey in 1920, became null and void, and the Treaty of Ankara of 1925, which involved the mutual recognition of the states, made no specific mention of the territories taken from the one the other in 1912-13. You can only say that the recognition of such annexations took place, and only implicitely, when Ankara opened a consulate in Skopje.
Also, the second Yugoslavia was expressedly built on a rejection of the institutional principles of the first, which had failed because it was a centralized state according to the Serbian political tradition. However credible under Communist rule, the Yugoslav idea was conceived of in Croatia in the 19th century as a voluntary union of equal partners, not as an empire where some conquered peoples were subject to recurring policies of extermination.
You wrote: for this reason, Serbians consider Kosovo the cradle of their civilization. But you gave no reason at all, and there is none: Raska, as I have said, was what is now known as the Sandjak, not Kosovo; the claim by some Serbs that their medieval state was born in Kosovo is nothing but a lie. The Serbs started invading Kosovo, at the expense of the Byzantine empire, after the Nemanjid state had become independent, and the conquest actually took place in the13th century. Is the above slogan supposed to mean that, by their own admission, they had no civilization before?
And then, in 1459, every Serbian state disappeared into the Ottoman empire. The Serbs may be the only nation in the world who have managed to convince otherwise sensible people that a 250-year possession by successive medieval states half a millenium and half a century ago is a valid basis for a contemporary territorial claim.
The truth of the matter is that the Serbian state invaded Kosovo in 1912 — and massacred 20,000 of its Albanian natives — after it had liberated itself from the Ottomans, because it wanted an access to the sea through the Drin valley (also, because it would help them hold Macedonia). All the Serbian myths now peddled about Kosovo were then fairly recent, as they are re-writings of history from the 19th century, including the claim that the first Battle of Kosovo was of decisive strategic importance.
Among these, the claim that Albanians "immigrated" into Kosovo is a formal absurdity. Since the Albanians are the descendants of the Illyrians, the first known inhabitants of the former Yugoslavia (except Slovenia), the Kosovar Albanians could not, logically, "immigrate" into Kosovo, all the less so when the whole region was part of a single state. They became an absolute majority again in their own country in the mid-19th century, when enough Serbs had left for New Serbia, the principality in the north which offered better prospects as it was de facto independent from the Ottoman empire.
The only instance of a massive influx of Albanians into Kosovo which actually took place is the one the Serbs won't tell you about: when 100,000 Albanians who lived in the region of Nis, Pirot, Leskovac and Vranje were ethnically cleansed by the Serbs in the winter of 1878. All the other stories about Albanians "immigrating" into Kosovo are Serb fabrications.
And when and by whom was the occupation of Serbia and Kosovo&Metohija ratified? See, no one ratified the illegal occupation of Serbia, including Kosovo&Metohija in the first place, so why should anyone need anybody ratifing something when Serbia including Kosovo&Metohija was liberated. Serbs liberated themselves and got rid of those who occupied them for centuries. And you think they should have asked the Ottomans for permission and needed some sort of ratification for doing so.