Even the first statement isn't a given: every step required a conscious decision on all parts. Also the Centrals made considerable gains on both their Eastern and Western fronts before getting bogged down in the West. In the East the continued war effort led to the Russian Revolutions. As the USA's involvement was starting at this time, the difference just wasn't enough to tilt the balance in the Centrals' favour.
At the time, it was indeed a given. Serbia and Russia were allies, and any invasion of the first by Austria-Hungary would provoke a Russian retaliation. That was a given. Russia certainly didn't want Austria-Hungary expanding its influence in the Balkans, it wanted to do so itself.
It was a very clever strategy on the part of Serbia to provoke a war actually, as it was blatantly obvious that Austria-Hungary could not defeat Russia (and they knew it, which is why they requested German support), that German entry into the war would result in France joining the war, and that Germany couldn't win a two-front war, which is exactly what happened. I doubt anyone expected that Germany would actually be stupid enough to enter the war.
That is where the miscalculation lay, and it was with Kaiser Wilhelm and his strategists, not Princip.
Sure, the Central Powers made gains before they were defeated. So did Persia in the 5th Century BC, and Carthage versus Rome. But they still lost, and deserved to, because they were outmatched. As the Central Powers were, and any unbiased analyst at the time could see it. WWI was the result of moronic nationalism on the part of the Central Powers. To coin a phrase, they let their mouths write a cheque their arses couldn't cash.
The involvement of the Black Hand is just one of the theories behind the assassination. Also, although Serbia attained its short-term goals after four years of world war, they could not have foreseen the outcome of the Versailles Treaty and the fact that the Centrals failed to exploit their Eastern gains to greater effect.
Sure, it's a theory, but it's almost certainly the right one. Princip simply wasn't bright enough to execute such a plan on his own. I think we can agree to that. He,
personally, was a bumbling fool. But he was a bumbling fool that got the job done.
And they very well could foresee the latter, although the complete humiliation of the Central Powers, including their local enemies, Turkey and Bulgaria, probably came as a very welcome surprise.
Now, I consider a "plan" that involves your own country being invaded and occupied to obtain some future goal by any standards a blunder; if it triggers a world war, in my book that lists it as one of the greatest blunders in history.
They probably didn't expect an occupation, but they knew there'd be an invasion, and that they'd eventually fight it off with Russian assistance; this happened. In that, they were successful. That the assassination triggered a global war was an unexpected bonus for the Serbs; after all, the Ottoman Empire's and Austria-Hungary's complete dissolution and Bulgaria's trouncing absolutely played into their hands.
And if you consider Princip's actions a 'glorious succes', you are following a terrorists' reasoning, which sees no consequences except the limited ones of its own aims. World history looks at it differently, however.
Why shouldn't I see it that way. Princip had a goal; that goal was reached, largely as a direct result of his actions. Hitler had a goal when he had the SA killed; his goal was reached as a direct result of his actions. Lenin had a goal: that goal was reached as a direct result of his actions. That all these men committed crimes, outright terrorism - particularly in the cases of Hitler and Lenin - is true, but that doesn't make their achievements any less real. They certainly didn't blunder in achieving them, they got what they wanted.
World history sees just about every thing as a blunder, if you look at it long enough. The wars of Louis XIV bankrupted France, and indirectly contributed to the financial disarray under Louis XVI. Therefore, Louis XIV was a bumbler who destroyed the French monarchy. That comment is patently ridiculous. He had short-term goals, and he achieved many of them, though not the ones he most wanted.
Likewise, Princip had short-term goals. He achieved them. That they haven't lasted, and that the results were horribly unpleasant for the region in the long-run, does not mean that he did not achieve his goals quite admirably in the short-term. In doing so, he succeeded beyond his wildest dreams. He most certainly did not blunder in the slightest.