That seems a little unlikely. IOTL the large powers apparently never considered partitioning Yugoslavia, due I suspect to the fierce resistance that would have met as well as to objections to , well, "balkanizing" the Balkans. Do you think Italy would have brought enough to the table to convince the powers otherwise?
If Benny play well his card can get a free hand in Yugoslavia as payment for declaring war against Germany/come back to the Stresa Front...expecially if Belgrade is a little too much cozy with Berlin; both Churchill and Chamberlain plus whatever will be the French premier will think of this as the lesser evil.