My youngest son's mother in law is old country Serbian. By coincidence so is my dentist.
I'll have my son ask his wife to ask her who the powers to be at the time in Serbia were, and what their wives' names were.
One name you left off was Victor Emmanuel III of Italy. Italy and Austro-Hungary disputed their border and who 'owned' Trieste from almost year dot. It's not unbelievable that Italy would back an assassination, let things kick off and wait to - hopefully - pick up a little territory. One could even say that was their motive for not entering the war until 1915 in much the same way Mussolini waited until the French
already had the stuffing kick out of them and the Poms looked to be on the ropes before declaring war.