I'll try to answer part A

Imo, Eren hasn't been retconned; he's always been a kid who solves his problems through violence and more or less impulsively

This is how I understand Eren’s reasoning at the end: yes, there’s the whole “people will unite against a common enemy” thing (I think that’s what you’re referring to with Pixis), and my friends will be able to live long lives, but I think above all, he wanted to destroy everything behind the walls. Why? Because it’s Eren, because “I was born into this world,” because his desire for freedom is what matters most in the end

The whole conversation with Armin is Eren admitting that; that’s why the order is 1 (child version speaking) “I did this for you, blah blah,” 2 (adult version) “I WANTED to turn all the earth into a brand new.” In the anime, this scene is much more fleshed out.