Very great story, I loved the slow build up. It was a nice change of pace, and they weren't snaky to each other all the time, and yet they weren't out of character. Your original character seemed just that and not a mary sue or a trite plot device, the love potion was a ingenious way to get around Ginny, not only did it get rid of her, but it explained how Harry went from seeing her as a sister to dating her over night, that has always been on of the few JK issues that never rang true to me. It was like she suddenly realized she had to give him a love interest. It was also a good way to have the whole issue thing explained away to Severus' satisfaciton, without him going into pages of suspicious doubts. As potion's master he would understand what a love potion could do, and as Ginny's potion's master he would know if she really had the ability to pull it off.
I also loved that McGonagall had the last word. I too want to say more, but this is a very satisfactory ending.