The inference is that it is pivotal, yes. That isn't to say there weren't other pivot points in addition to Garak's intervention here. I like that episode because it's easy to forget that with Garak's slippery charisma he's capable of brute acts to achieve concrete aims uncumbered by the kind of handwringing our heroes get up to.
Nope. Sloan is the one most responsible for beating the Dominion. Or at least Section 31. But the whole infect-the-Link op seems to have been his baby, and that is what caused the Dominion to fold.
Why is that what would have happened? What DID happen is the Founders threw in the towel. Not any qualified form of victory. A victory.
The Dominion has already been defeated though i.e there was zero chance they where going to take the AQ. The founder capitulating simply decided different degrees of carnage, the war's outcome had been decided by then. It looked odds on the AQ would fall before Garak's intervention secured Romulan support however.
As I recall, Odo carried the cure, which is why he had to go back, so all he was was the vector for introduction of the cure into the Link. Also, if the Dominion is as powerful and resilient as some here believe, then I very much doubt that it would have given up in perpetuity just because it couldn't get ships through the wormhole. I can see the Founders doing anything to get at the AQ, even taking the long way. Or if they are such advanced scientists, finding another wormhole into the AQ. Bound to be some. Or waiting patiently until they DO find a wormhole (and I can imagine the Dominion bending every resource to find one) and attacking then. This way, it is over, done. At least on paper, if you believe the Founders' word can be trusted.
The problem is that there was no one turning point nor one easy solutions. Behr and the writing staff consistently undermined anything that looked like a magic bullet or a final blow, making sure that in the end, the war would only be won by effort and cooperation.
