[3] For getting chosen over Erwin
First of all, Erwin and Armin had very different goals.
Not sure we're reading the same manga if you're hating on Armin for this stupid reason. Like stfu, you don't even know their characteristics. Erwin himself, doesn't want to be saved so respect him and get tf outta here.
Anyways, Erwin himself didn't want to be chosen. He knew that in order to secure a better future for humanity, his sacrifice must be made. He was a very strong and courageous soldier indeed. He was special to humanity. We all know that. But, he's doing it for his own goals. Which is to learn the truth about titans regarding in Eren's home/basement. And after that, since his purpose is fulfilled, there will be no purpose, no nothing. The purpose of his life would be over. Levi wanted to pick Erwin, too, but he considered the consequences and events, and he himself chose Armin. Meanwhile, Armin wants to give humanity a better future. His purpose is far more important than Erwin's. Levi even said "I have no regrets for not choosing you, Erwin, for choosing someone with the same look in his eyes". Meaning, Armin is the key to humanity. They NEED him in order to bring peace to humanity, in order to end the wars and stuff. Eren even said "The one who will save humanity is not me nor commander Erwin. It's Armin". (Also, Erwin was the one who saw the potential in Levi. He basically raised him. So even if someone with strong affection towards Erwin would choose Armin (because of logical thinking), you shouldn't budge in)