Not really. It took a full month for the austrian to send Serbia its ultimatum. If the ultimatum was given imediatly after the assassination, both France and Russia would have let Serbia go.

Also, in France Caillaux, one of the most powerful pacifist in the government (he had the economy and was _good_ at it, which is rare enough to be noted), who was anti-war, had to be let go because his wife killed a journalist (not really a journalist, but adjascent enough so that the difference don't matter) around the same time, leaving Jaurès alone to push Viviani to let it go. Plus Viviani was with the Tsar in Russia when the ultimatum was given, which surely did not help _at all_. Especially since Rasputin was out of the capital at the time. Truly an unfortunate timing here.

Plus the death of multiple diplomats involved in the prevention of WW1 at least once in 1911, who all decided to die around the same time (the last two in 1914, Pressensé and obviously Hartwig, whose death was truly, truly unfortunate).

And here i'm only aware of French people who would have prevented the war (beside Hartwig and Rasputin, but everybody know about them), but i'm pretty sure even more people in Germany and in Austria could have done the same.