Schlieffen was the best option they had.
They rightfully assumed that they couldn't beat Russia quickly, due to its sheer size.
They also knew that Germany couldn't afford a long war, due to lack of ressources within Germany. (especially Iron)
So the only option was to attack the smaller (in size) country with full force and taking it out as soon as possible.
They miscalculated regarding Britain, which was the blunder that in the end lost them the war, though the British didn't have that much impact in August 14.
Biggest additional mistake was to weaken the plan, by weakening the right wing in favor of the left before the war and by pulling out 2 Korps and sending them to the east in August 14. (They were desperatly missed in the west and were too late to fight at Tannenberg in the East)
Depending whether a victory at the Marne would have changed thing: Yes. Paris is the heart of France in every way, it would have been to huge a blow to moral to lose it. Not to mention economical and logistical problems it would have caused.
Yes, i know they wanted to defeat France as quickly as possible and then turn on Russia, but they would still have the UK and commonwealth able to strike them in the back, and if they would try to strangle the UK into submission with submarines they would just bring the USA into the war. Thus basically a replay of WW2, not sure if that would be great.
Wishfull thinking reigned surpreme I guess, they still hoped England would remain neutral even after they invaded Belgium.
Hell they even tried to bully Belgium inton giving them Military Access, which of course Belgium wouldn't do as they didn't want to oppose France.
Times were different in 1914-18 though. Amphibial operations weren't as far developed as in the 1940s (see Gallipoli), Air Raids also weren't really effective, so the UK would have sat on its Island unable to do anything in Europe. Same problem for US troops, if they had joined.
