What does Germany hope to achieve?
Stabilizing A-H by giving them cover to crush Serbia, cut off Russian ambitions in the Balkans, and smash up Russia so they don't outpace Germany in terms of military power, which they were set to do in a few years; Germany was interested in maintaining the balance of power in her favor and she got a chance to do so when Russia mobilized against A-H. Had Russia not, then Germany could sit back and let A-H wipe out a major source of instability and ensure its long(er) term survival.
Thats the basic point. Noone in 1914 believed or was organised for a long war. The entire point is it would be violent, decisive and short. Thats the experience of the Balkan wars ( less so russo japanese, but that from the Russian point of view at least was more like an embarrassing colonial and naval war.)
That's somewhat false; Germany though the war was going to last into 1915 and then a negotiation would end it. Britain too planned on the war lasting into 1916. Russia and France as far as I can tell didn't think it would last years, nor did A-H.
What you are trying to sell to Germany is a strategy of exhaustion that at best assumes the other guy will collapse before you will (and both would happen quickly) and at worst a multi year war that both impoverishes the nation even in victory and assumes Russia or France will collapse before you do.
To a degree yes, but they anticipated that they would be able to smash Russian and French military power, which would keep them as the principal military force on the continent, rather than the Franco-Russian alliance getting any stronger and overriding A-H (which was weakening) and Germany (which was reaching its power limit). This plan would be predicated on winning a few decisive battles and knocking out the principal source of trouble, Serbia, while convincing the French and Russians that they would lose or at least not win, so better cut a deal in the short run. It would be a prisoner's dilemma situation once Serbia was crushed and Britain was neutral (and the Ottomans in the war), either they could cut a deal and end the war or bank on an exhausted stalemate; so Germany could run up points by winning a few important victories with A-H and wait out their enemies; attritionally Germany and A-H can win in the long run once they gain certain objectives. With the Ottomans in the war Russia is going to collapse economically with the Dardanelles closed and France will be broke by 1916. So long as Britain stays out and the CPs don't do something stupid like invade Belgium, they had a winning hand after 1914. Just don't make any mistakes and your enemies lose.
A plan to attack in the east based on pre war knowledge is a plan for an indecisive war thats costs a fortune in lives and treasure. If thats the option being put forward - why go to war?
Because it settles the Serbian question, cuts off Russian ambitions in the Balkans, and weakens Germany's enemies more relative to the CPs.
Why not just mobilise and defend Germany with a threat to attack Russia IF they intervene in Austria's justifiable action against the Serbian terrorists?
Because that let's Russia mobilize completely, which is bad news for Germany; the reason they went for quick victory in France was to defeat her before Russia was fully ready to fight; here without Germany declaring war on France she would anticipate beating several Russian armies before they were all ready, which would fulfill the beating the enemy in detail before his full strength was drawn up. As it was Russia ordered mobilization about A-H and if Germany mobilizes then Russia mobilizes against Germany and attacks A-H, who cannot beat Russia and Serbia together on her own. So Germany has to get mobilized first, beat some Russian armies, take the pressure off of A-H so that they can complete the defeat of Serbia and bring their full weight against Russia, and then go for the big combined win against Russia by holding important territory in Poland, the Baltics, and Ukraine while letting the Russian economy collapse and France, if it declares war, will bash itself bloody in the west.
If you want to come up with a plan to attack Russia in 1914 do so, but do it on the basis that you will have to destroy Russian armies not on an assumption that they will just conveniently vanish.
That's what the plan was, defeat Russian armies, grab important terrain, and wait for A-H to finish its job in Serbia so that they can fight the rest of the Russian armies when they mobilize together.
Remember also if the enemy has 3 options available to him he will inevitably choose the 4th.
Which would be?