I tend to agree here. Also, as Serbia's expansion was huge anyways, Italy occupying a few ports during this phase might not bother other that much.
That's just the thing. Serbia outlined a "Greater Sebian" programme in her declaration of war with Austria, claiming she would liberate Serbs and listing all the places they were to be found, but saying nothing of Croats and Slovenes (quite understandably, since the racist "Srba na Vrba!" chant originated in 1914 as a translation of "Alle Serben muss sterben!": Croatia and Slovenia were generally pretty pro-Hapsburg). King Peter as pretty suspicious of the whole idea of "Yugoslavia", and nobody had seriously raised it in 1915 when London was negotiated.
The London treaty outlined both "Greater Serbia" and Italy's Venetian claims, overlap being fairly minimal. This left Croatia with pretty much nuthin', and the Croats and Slovenes were scared out of their wits at the prospect of the Entente winning and rallied again to the Hapsburgs.
Yugoslavia was formed for a variety of reasons (Wilson's support, for instance, and the distorting effect the exile of the government had had on Serbian intellectual and political life), but it was principally because the South Slavic elites of Austria-Hungary (and you could meaningfully talk about "South Slav" elites there, because the Croats and Slovenes often co-operated toward common goals and in this case the local Serbs were also in agreement), having seen that Austria was going under, turned to Entente-backed Serbia for protection from Italy. The fact that nobody had ever really considered the logistics of uniting two territorially mixed and bitterly sectarian populations in one state for essentially tactical reasons led to the somewhat frantic state of interwar Yugoslav politics; and during that period the only thing that could really unite Serbs and Croats was the Italians making noises.
You could say the intellectual origin of the Ustasha was as Croats who for whatever reason (because they held Catholicism and sectarianism above Yugoslavism, or just because Yugoslavia was what they grew up in as young radicals) would rather Italian sponsorship than sharing a state with the Serbs on any terms.
So the point is that it isn't a matter of what the Belgrade government wants, it's a matter of what the local inhabitants who control the old Hapsburg institutions want.
As already stated, the political situation is too fragmented. You'd need either to butterfly or co-opt the Red movement or have some external trigger. Note that in 1918 you already have a "Versailles Yugoslavia" being built over the very ground Italy wants.
Maybe if the burgeoning Yugoslav state sends troops to "liberate" Fiume from D'Annunzio you could have casus beli enough for a war with Yugo, which at this point will be a hodge-podge of Serbian and former A-H Slovene and Croat troops and generals who may not trust one another versus a war-weary and politically fragmented Italian army in territory far from good for combat. Ugly, ugly, ugly.
Indeed. The "State of Slovenes, Croats, and Serbs" was pretty much a continuation of the Austro-Hungarian state in those areas that was grafted onto Serbia, led by the South Slav parliamentary caucuses. Vienna even formally handed them the navy, to make sure the Entente combatants didn't get it.