Blainofsky could work as a pairing if it weren't for how much their relationship surrounded Kurt, like had Kurt truly not been a part of the equation, had Blaine been properly over his relationship with Kurt, him and Dave could have worked.
Dave was really sweet with Blaine and unlike in his relationship with Kurt, Blaine and Dave are on different career paths, they aren't ever going to be in competition with one another and that is one of the things that killed Klaine before. [Among other issues]
Blaine needs someone supportive but also someone that doesn't have to put themselves down to be supportive, someone like Dave.
In the same way, Kurt needs someone that is going to push him to be better, to strive for his dreams and allow his snarkier personality shine, not someone who is going to downplay or even discourage his improvements and growth. He needs someone like Sebastian, and like I know Sebastian was mean to Kurt, but if Kurt can forgive David 'Death Threats and Forced Kisses' Karofsky, then I think he can also forgive Sebastian 'Insert Any of His Quips' Smythe.
Now Sebastian being attracted to Kurt? I honestly highly doubt Sebastian was ever actually into Blaine, he was playing a part to get Blaine back to the Warblers, he was dropping lines he knew would work, he had no real interest in who Blaine was at all.
And S3 Kurt wasn't going to catch anyone's attention, unfortunately. When Sebastian and Kurt met, Kurt was this anxious, unconfident version of himself. He was a version of himself that had been tormented by bullies, had watched his crush turned boyfriend flirt with/crush on every possibility [even girls] before even considering dating him. He had been laughed out of his audition for being too 'girly'. He wasn't himself and he wasn't the type to catch attention. Frankly Kurt was an outlier to Ohio, leaving that state was the best thing for him because it allowed him to thrive.
But NY Kurt? Kurt in his element at Vogue? Kurt who had everyone in Stage Combat looking at him? That would be a Kurt with confidence, and a Kurt with confidence would catch a many peoples' eyes.
