Again, you guys are vastly overestimating Italian offensive capability. At no point during this time did Italy demonstrate ability to conduct offensive operations of this magnitude with any degree of success, and just because Yugoslavia bordered Italy during this time would not make it an easier task. The supply lines will be very vulnerable - Yugoslavia of this time does not have a highly developed train and road network, and would have a very hard time pacifying the occupied territories to secure the supply lines (terrible as they were). Their best chance would lie in convincing Croats and probably Slovenes that they were less of a threat than Serbs, but in TTL this is almost impossible (even in OTL NDH, although nominally an ally of Italy, had a deep mistrust and dislike for the Italians).
So how does it go for Italy in this invasion? Worst-case scenario they're immediately stopped in Slovenia, best case scenario maybe they reach Dinara and Velebit ranges, but can't seem them going beyond that. In the latter scenario, maybe they hold some coastal areas they wanted and manage to annex them, but that the very best outcome they could've hope for, and far from certain.