1. Nobody would be prepared for war in this instance. Pretty much every country's warplan at the time involved intricate mobilization plans which took advantage of the ultimatums and diplomatic wrangling to come into effect. If the war just suddenly starts, then none of the major powers is in a position to carry out its warplans expect with what forces are actually deployed on the relevant borders, which will hamper the conduct of the war. Even asuming that the declarations of war are roughly the same, each country will be frantically mobilizing and either trying to strike before their opponent can or finish preparations for an actual war. It is quite likely that by the time any country sorts itself out enough and is sufficiently mobilized to go on the offensive according to their original plans, the rest are ready enough to stand them off, and stalemate occurs much sooner on most fronts.
2. Diplomatically, Austria loses. Yes, their heir was shot and killed, but then suddenly the AUstrian army pours into Serbia on a flimsy justification and with no warning. A lot of people are going to be taken by surprise, and many will not be particularly thrilled by this course of action. It is possible that the allaince structures shape up differently, or that Germany declines to support Austria, thus aborting world war one.
3. Ultimatums were how diplomacy of this type was carried out at the time. It was pretty rare just waltzed into a neighbor without some sort of protracted warning or diplomatic incident. Due to a mix of military and diplomatic factors, the idea of a sneak attack of the sort you polstulate would be dismissed out of hand by many in positions of power at the time.
BTW, this goes in after 1900
