How is it possible for you to disagree when the characters outright tell you this is how the system works. It's black-and-white in the dialogue. There's plenty of things in PMMM subject to interpretation, but this is not one of them.
>>as Kyubey puts it, "there's a curse growing inside her".
That's not the "start" of the slope. That's the nature of the slope. The mechanism of the slope, if you prefer. Not a definition of its beginning.
>>purify their soul gems or else they become a witch, which is a slope. But that slope becomes slippery when she begins to despair
I see those two as the same thing.
>>Which happens when Hitomi and Kyousuke get together.
Yes, that's an example of her going further down the slope. And in the timelines we see it's certainly a major part of her descent to witch-hood. But is it the ONLY cause? Based on what the various characters have stated, we must conculude that it is *A* cause, not the only possible cause. The moment a girl contracts everything bad that happens is a step further down the slope. Clearly some steps are larger than others, but any and all sources of despair pushes them closer to witch-hood. If Sayaka got over Kyosuke then she'd simply be delaying her descent into witchhood until something else did the job.
That's the whole point of the show, really. Once contracted, witch-hood (or death) is inevitable. The characters state this over and over again. That's why it's so tragic—no matter their good intentions or how hard they work they can NEVER escape the fact that once contracted witchdom (or death) lies in the future.