Frankly I too believe that Italy will likely become a non-factor. A significant part of their war effort depended on French support (although less than British and American support was rising) and France also provided 2 divisions to the Italian front from 1918 onward. Italy had a significant manpower shortage itself (look up the Ragazzi del 99). Even otl with significant reinforcement from the western front (a bit more than five divisions in total) the Italians needed until October 18 to feel strong enough to attack AH. A further ca. three divisions were raised from POWs in 18 and I suspect that effort will be significantly less successful if they think association with the loosing Entente will damage the nationalist causes they aim for (depends on the exact timing, the first Czech units in Italy were formed in January 18, but the Legion was not fully consolidated until October). A temporary drop in strategic supplies and a significant delay in urgently needed reinforcements (French and at least part of the British forces coming otl won´t be availabe ittl) make it likely that Italy will sue for peace once France is out.Italy hasn't fallen.
The issue is the Alps and the ability to supply forces forward. Germany would probably spend the rest of 1918 wrapping up and occupying/pillaging France, while dealing with the Brits and Americans before being able to move on Italy. In the meantime the Italians would probably go on the offensive as per OTL and drive out the Austrians and maintain an Alpine frontier.
That said I think that Germany will accept (to a degree might even welcome) a dissolution of AH and likely will try to guide it. My gut feeling says they will favour the Austrian approach of a soft division along national lines over the Hungarian goal to keep the two halfs intact by force as two separate states. How soon they come to the conclusion, how they will try to influence it and how successful the dissolution will be is another question though.