After watching through a few WWI videos and educational videos I keep coming across part of the story that confuses me: As part of quelling Hungary's discontent with the war and their fear of losing their new rights, they agreed to not annex Serbia or even demand land concessions.
The part that confuses me is, if land wasn't on the table, what did Austria-Hungary want from Serbia? Monetary reparations could never cover the cost of mobilizing their whole army, forcing their police in seems a very minor demand for a full scale war, I just cant think of anything they would want.
I'd like to see the sources that you watched or read that stated they definitively agreed to no land concessions. Prior to the beginning of WWI, the Hungarian head of state was prime minister Istvan Tisza and although Hungary in the past concerned with any Austrian-Hungry expansion that could incorporate a third party to its empire, I see little reports that Tisza made this argument. He was against going to war with Serbia initially and was one of if not the only proponent of a purely diplomatic solution but after some negotiations he seems to soften to the idea of dividing the majority of Serbia among the other Balkans states and creating a puppet government in what was left of Serbia. This would solve the problem of an unruly Serbian neighbor while still appearing to not bully a Slavic country that could anger Russia. It is said that late in meetings among the Austria-Hungarian council Tisza said he was "anxious to meet the others halfway and was prepared to concede that the demands addressed to Serbia should be hard indeed, but not such as to make our intention of raising unacceptable terms clear to everybody else." Here we have the Hungarian prime minister stating that he could agree to demanding terms from Serbian that they would be unable to adhere to, as long as it was not obvious. The primary fear was that Russia would get involved in any conflict with Serbia if it did not seem that they were the unreasonable party.
So to answer your question, from the very beginning they intended to dismantle Serbia and create a smaller Serbian puppet state in which Austria-Hungary would control and the demands that Austria-Hungary made of Serbia were meant to be particularly harsh to the extent that they would not be acceptable. The demand you allude to, that Austria-Hungary be allowed to conduct the investigation with their own police was one of the demands that Serbia refused.
Sources :
Z.A.B Zemans The Gentlemen Negotiators: A Diplomatic History of the First World War
Alan Sked The Decline and Fall of the Hapsburg Empire