1: So rarity can be in love, but Twi can't? WHY???!?!?!?!
Rarity had a crush on Prince Blueblood in Season One, so this is fairly normal for her fairly dramatic personality and fans recognize the similarity instantly. Twilight is allowed to have a crush, fans would accept a stallion if it was one worth accepting. they don't like Flash because of his stereotypical highschool coolguy personality
in the end, she wasn't going to be involved with the guy anyway because as we all know: nothing changes. even when you become an alicorn, nothing really changes
they keep taking breaks from the season's supossed main plot of those rainbowy things and introducing these events in ponyville that were never mentioned before just to make a completely off topic episode about a character we'll most likely never see again?
because this series is known for its standalone adventure and slice of life episodes. the main plot of rainbow things is experimental and only related to a few episodes (premiere, finale, six or five episodes that give their their objects). it's not a 26-episode plot, it's less than a half-a-season plot.