Didn't we have the same thread a month ago? That's not a mistake, btw, just a strange coincidence.
There are a few problems with this premise. Like the idea that the Serbian government was involved in the assassination.
But, even we choose to sweep those problems aside, there's still one major problem.
Even if the Serbian government was actually involved in the assassination (which is a very controversial and unlikely idea - to put it mildly)...
...and even if Austria-Hungary found some kind of document revealing this...
...no one would believe them.
Why? Well, during the crisis of 1908-1909, there was a series of trials in Austria-Hungary. In these trials, the Austrian government tried to use a bunch of fake documents as an excuse to persecute the Serbian minority and prepare for a (potential) war against Serbia. However, the attempt blew up in A-H's face: the documents were outed as fakes, and there was a humiliating international scandal.
In other words, everyone knew that Vienna was not above using totally fake documents to push its agenda (especially when it came to Serbia). The Entente would likely see Vienna's "proof" as nothing more than another forgery.