You have some points. And I do see where you are coming from. But I don't see how the short flash of Stacy and Logan together could be counted as evicence. And Mark hurt Quinn. Badly. I don't think she would go crawling back to him.
And I don't think Stacy really knows what she wants. Why would someone love a person so much when they are so cruel to them (and not in that, 'I'm being mean to you now, but you know I'll always be there for you' love-hate relationship way) ?
Don't get me wrong, I want Stacy to be happy, we all do, I think. But Logan just doesn't feel like the guy who'll bring her that happy ending.
And you make some good points as well.
I don't see how that clip of Stacey and Logan can be explained away though. This is the finale of the whole series. It's going to be about tying up all the loose ends really (I recall Kathy Sanders saying that all the loose ends are tied up in the finale, or something to that effect). I don't see how they'll have time to throw in stuff simply to throw us off. Let's look at it this way. If Quinn and Logan didn't get together yet at all in this season, and you saw that clip of Logan and her together rather than Stacey, how would you react? I bet a lot of people would say they are together. That clip may have been short simply because it gives a lot away. To me, it is good solid evidence of them possibly being together. I don't see how it could be any more concrete.