Agree the Serbian military were deeply involved in the assassination, via the Black Hand, but not much evidence that the Serbian Government had much knowledge of what was going on. One of the causes of WW1 was the lack of political control of the military of many Central European states, most crucially Germany of course, which by 1914 was famously ‘an army with State attached rather a State with an army’.
Why did Austro-Hungary feels so emboldened in their reaction to the assassination? Only due to the ‘blank cheque’ telegram of support in terms of German support for whatever action Austria-Hungary chose to take in punishing Serbia for the assassination sent on 6th July and stressing the need for immediate action.
Without Germany pushing from behind Austo-Hungary would have been unlikely to issue such strong terms to Serbia, and very likely would have accepted the Serbia counter-proposal which accepted 95% of what were generally consider outrageous demands. The conflict in the Balkans had a good chance of remained localized, just like several previous Balkan Wars, without German interference.
By 1914 German was just looking for an excuse to start a European War, and was not going to let the one that emerged go to waste.
Everyone except Serbia, Belgium and Britain wanted a war. It was only a matter of when and on what pretext one country or another managed to kick it all off.
No, this is a postwar myth. Most of the countries that formed alliances really were doing it for defensive purposes. They felt that by forming a strong alliance, they would deter the outbreak of a war. Most of the countries in Europe really were trying to avoid a war.