Uhh regarding Armin and Erwin, no one seems to be talking about the fact that Armin is more future-facing than Erwin. In fact Armin and his ideals/dream are the future. That was the real turning point for Levi's decision, when it flashed back to Levi hearing Armin talking about the sea. Erwin just had the basement and knowing the truth in mind, but Armin's trying to know more about the rest of the world on a different sort of level.
Basically Erwin is the old guard who's death means things can be better for the next generation.
Also as a side note, Armin is arguably a better person to make use of the Colossal Titan's powers if need be. Erwin is a commander so he's not going to be up in the front lines as a Titan. Or at least he shouldn't be. Armin has a quasi commanding role but it's not the same.