Mewtwo never erased Ash's memory, he erased everyone else's. He let Ash keep his because he respected Ash for showing him there was hope for him in this world. Doesn't explain why he doesn't know what mewtwo is in the B and W movie though. As for those eevees, they were content and evolved in the episode. Those two things are why he didn't go after them. He only will catch something if it approves. And whenever something evolves in the anime, they don't catch it. Ash also has seen eevee twice before then and didn't want one, so he may either want someone else to catch it in those episodes, or he doesn't want to catch them. The consistency is weird though, as Ash first encounters and wants a dunsparce in Black and White, and then in X and Y he finds another one and wants to catch it., and doesn't due to other reasons. I hope he catches one eventually.