The scenario you suggest is unlikely.
For one thing, Montenegro voted to join Serbia directly before the founding of Yugoslavia. That's probably still going to happen, once King Nick discredits himself by looking for peace.
Ad without Yugoslavism, are we sure there's be a Slovenia? The Slovenes were, IIRC, the most loyal of all Austro-Slavs, and staid so until very nearly the end. And of course owing to Italian claims a post-WW1 Slovenia would be landlocked and in a position to be very pressurised by Italy... I can see them sticking with Austria is Serbia's not interested.
King Peter was apparently a Serb nationalist and not much of a Yugoslavist at all, but Serbia would, assuming any kind of Austrian breackdown, demanded Slavonia, which was promised under the London Treaty (a document which did not foresee Yugoslavism). The treaty also gave Dalmatia to Italy: we've got a pretty neutered Croatia here.
There are various ways to get some variation on this without a CP victory. A compromise peace could give Serbia Bosnia and Srem, and then Austria fragments shortly after, leaving an intact Croatia to spin off.