It's not exactly correct to say that the Kingdom of Serbia chose to create the Kingdom of Serbs, Croats and Slovenes after the conclusion of the Great War. While certainly the whole process could have fallen through, the name of the new state itself implied an agreement between the three national groups reached on an enqual ground.

This agreement had been, after a couple of years of complicated talks, reached in substance with the "declaration of Corfu" signed on July 20^th 1917 between long tenured Serbian Prime Minister Nikola Pasic and the leader of the committee of the "oppressed yugoslav nations" within the Austro Hungarian Empire, Ante Trumbic. It accepted the form of a constitutional monarchy under the Karageorgevic dinasty, even if it could not for obvious reasons go through any proper consultive process (such as a popular vote that had been advocated by a few - notably the Croat F. Supilo - on the possible federative nature of the state). The main reason for this internal debate had been the traditional special position of the Croat nation within the A.H. Empire, that led some of the Croat establishment to still look at the possibilty of the creation of a "federation of crowns" within the reformed Empire - a project still very far from coming to fruition, but something that had been up in the air for a while (and was in substance a much stronger version of the Archduke's plan).

To better understand this point, it helps to look at the political sistemation of the Dual Monarchy as of 1914 side by side with the territorial promises made to Italy with the Treaty of London.

Incidentally the Italian intervention took place in May 1915 and was negotiated during the early months of the year (it would have made close to no sense by 1917, as a large portion of the Italian establishment was becoming to realize by then). The Treaty was in fact fairly detailed in terms of the Italian expansion and, as you can see, for the purpose of the Adiratic region, it mostly involved the occupation of the Istria peninsula, major and minor islands of the coast and the northern portion of Dalmatia. It notably excluded the southern portion of Dalamtia as well as Fiume (yes indeed!).

The reason is not difficult to grasp: by 1915 the activities of the Yugoslav Committee were far from well established - there were still substantial internal divergences and both Trumbic and Supilo had yet to gain a concrete influence with the Entente Powers - and for the three major powers, France, GB and Russia, securing the Italian intervention was by far the more pressing concern.

For this reason Russia - which was at the time an (almost unwelcome) supporter of the pan-Serbian plans for a Greater Serbia had been the one nation to insist on the expansion of Serbia towards the coast. The Russians believed at the time that a Serbian Kingdom would have been easier to position as their own satellite in case of a victorious conclusion of the War. For this reason the southern portion of Dalmatia had been reserved in order to provide - together with a portion of Bosnia-Herzegovina - Serbia with an acess to the Adriatic Sea.

The other exclusion had been made for similar reasons. In fact, despite us being used to think of Croatia as a nation with a large portion of coast, as of 1914, Croatia was an almost landlocked territory. Traditionally a vassal of the Hungarians, it had a peculiar sistemation as the one "old autonomy" province of the Hungarian part of the Dual monarchy (I probably should count Transilvania here as well...). Historically, the Austrian crown had played strongly on the Croat ambitions to keep in place the Hungarians, at least since Jelacic's declaration in favor of the Emperor in 1848, and the Hungarians had been trying to restrain the autonomy ambitions of the Croats in a similar manner.

Now, what we need to understand, is that the Treaty of London presupponed an Austro Hungarian defeat, not an Austro Hungarian dissolution. Russia, France and GB had approved of the Italian expansion but - even in the realm of realpolitik - they had not power to "grant" lands they did not hold. The Italian foreing ministry, A. di San Giuliano (and later his successor S. Sonnino) had therefore laid the ground to make the Austrians more inclined to accept the treaty in case of an eventual defeat and negotiated peace. They had left out the Croat coast - that the Hungarians would have liked tied to themselves ever more closely - as well as the Port City of Fiume which the Hungarians controlled directly. Their idea was in fact to leverage on the Hungarians' own autonomy ambitions and on how the threat of an Hungarian secession might have played in the negotiations with the Austrians.

It was certainly a dated approach but a coherent one - especially given the traditional good relations between Italian and Hungarians established already in the first half of the XIX Century.

Now, fast forwards two years. Russia is soon to be out of the war and certainly unable to exert any diplomatic pressure. The conflict has led to the occupation of the entire Serbian nation. The perspective of a negotiated peace appears ever more unlikely. The American intervention has brought forward a new theme of the "eight of the people" and self determination.

In this context it became apparent that a solid agreement - even with a few concessions to the other side (for instance the Serbians having no veto power in the new parliament or the Croats accepting a principle of a representation more or less based on numerical consistance) - for the creation of a unified nation (Pasic's "large plan") could be more productive than one based on the "little plan" of a greater Serbia.

As for the purely speculative part of your question, there's really no way to answer a "what if?" properly. The two parties had in substance accepted the sistemation and it is clear that there was concern over a possible Italian attempt to force a favorable solution (the negotiations involved already some military arrangements to protect the border - albeit "from the Germans"). Notably though, the Italians never intended to acquire what the Austrians recognized as Croat land - only the portion of the coast directly controlled by the Austrian Crown - so, to the letter of the Treaty of London, Croat and Slovenes and the Dalmatian coast were different things. The annexation of Dalmatia was an issue for the Croats since a good number of residents were recognized as "ethnic Croats" and they feared (understandably) a process of "de-ethnicization" in consequence of emigration from those lands; but Dalmatia was not a part of Croatia in the 1914 sistemation, with the exception of a very minor coastal trait that the Treaty of London did not touch.

My main source has been L. Valiani's La dissoluzione dell'Austria-Ungheria.