I agree there are way too many assholes in the fandom.
On the subject of the inconsistencies,
1. Doesn't Twilight keep on refering to Nightmare Moon, as the Mare in the Moon in pilot, so it is possible that part of the legend was lost over time, it does happen IRL.
2. Twilight was an asocial bookworm, when she attended Celestia's school, so it's very possible she didn't go out much, so it's possible the upper crust of Cantorlot don't know about her. Also IRL, people seem to have short memories, so according the show it seems to be the same with ponies.
About the origin of the fan base it may have started with 4chan for many, but I'm sure many have different stories, I've never been on 4chan, and I started watching it out of nostalgic reasons(I loved G1 MLP as a kid) and if it wasn't for my mom telling me that MLP was on, I probably would have not found out about it until later.