The only side Marlborough was ever really on was his own. He was very good at seeing which way the wind was blowing, making sure he was on the winning side, and yet still keeping fingers in enough pies that if that wind were to shift he'd still be in a good enough position with his new masters to maintain his position.
Hmmm... Marlborough according to the conspiracy was supposed to lure government troops to him by making an overt move (because most of his troops are more loyal to him than the actual government... complicated story. Not to say they had no national pride, but they thought their commander knew who was the right king). Then The Old pretender was supposed to give a commission to one of his supporters, and a march was supposed to be made on London. James Francis Edward Stuart ("James III") never got back his reply, but was sure that given Will and George's supporters screwed over Marlborough's allies (pawns?) was 100% sure the two of them were on the same page. After all, he had correspondence with Marlborough's household in up to 1712 (not Marlborough himself, but the servants supposedly were writing on the behalf of him). Then there was a communication blackout (conspiracies need to stay hidden) and when Anne kicked the bucket The Old Pretender sent his "OK, I just need to ready my supporters in Scotland, you know what to do" to Marlborough and thought they were on the same page.
The Earl of Mar raised the standard of war before The Old Pretender could organize his other conspirators. If he waited another 3 months before doing anything to alert the government (like... uprising), Mar could have tripled his numbers, get some wagons and horses, and "James III" would be on his way to Scotland to act as a symbol of their cause. Fortunately for Mar, the government was so unprepared that after the Battle of Sheriffmuir, there was nothing between him and London despite lots of Jacobites that could have oined not joining. Forutnatly for the government, as Neil Oliver said Mar knew how to parade, not fight wars. So he paraded around Scotland while
Now this is the story that pretty much went uncontested as late as Victorian times. Mar rough heard about the uprising in Scotland. He dispatched three small cavalry squads to look and asses the situation since he had not been given orders by George I to move out and he was unwilling to waste government money marching and feeding when he didn't even know where the rebels were or if his help was even needed. The squads split up, supposedly because if one group got defeated by the rebels, the other two might spot the rebels and flee south. When the scouts came back to report after Scheriffmuir, he realized the uprising was serious, but still didn't know there was a road open for the rebels to march into London (as the government forces were not only unprepared for an uprising, they actually denuded three of their garrisons on a critical road for the field army). Thinking other government forces were putting on delaying actions (because you're an idiot government if you haven't mobilized your army in November when London heard the news of the rebels 30 days earlier!), he marched to secure London at a leisurely pace. Since there was no way the rebels could advance fast with even a puny 3,000 men delaying force (which the Hanoverian British could no longer muster to put between Mar and London in the time it takes to march there thanks to the Scheriffmuir battle and them unable to mobilize other troops with any urgency), he marched slowly to avoid tiring his men and taking on unessary casualties (friendly fire goes up when everyone is hungry and sleep deprived) After getting north of London, he heard more news about actions between rebels and government forces. Marlborough then proceeded to split his guys into three groups and help with mop-up operations in December.
The complete lack of urgency in mobilization is either a perfect example of what not to do or parliament and cabinent (which was where the bottleneck was, not the army units themselves) was subconsciously not caring about a Catholic Scotish or a Protestant German on the throne.
In the 45, the Jacobites were faster and more organized, but had less of a chance because the government was not made of morons this time.
Marlborough said he was a Georgite (not his exact words, mine) and had sent a letter (HE did, not his servants in his name) to the Hannover to congratulate George on his ascension. He had made assumptions on the government actually doing something given how much lead time they had to react and his officers praised him for his decisions made with information he had (keep his men well rested and fed). Few people critized his assumptions, which were perfectly reasonable, and blamed the government for not communicating with Royal Army barracks fast enough (as they should!)
Three years later, he found correspondences between late members of his household and "James III" and denied any involvement in the plot, saying his servants had tricked the Old Pretender.
If what you claim is true, he once the Hanoverian-British lost London, he would twiddle his thumbs instead of storming to retake London. Are you sure about that?