About question 5, if you do decide to do that plot-line, I feel that the right alien for Izuku should be Atomix. However unlike Ben who became Feedback because of the sudden rush of power that Feedback gave him, Izuku’s addiction stems from his fears of not saving everyone and winning his battles, in addition to needing a “release” from himself.
In your story, you could introduce an event that caused Izuku to lose faith in his abilities. He essentially reverts to his middle school personality. He’s anxious,insecure, mistrusting, emotionally-sensitive and lacking in self-esteem and confidence in his skills. He’s hesitant to respond to situations that involve physical confrontations but will rush in when he notices that someone is in danger. Even so, Izuku lacks the confidence to win his battles or save others.This lack of self-belief self-belief starts having a detrimental effect on his aliens. His dwindling fighting spirit has weakened his aliens, the lack of purpose in his strikes, the absence of purpose in his movements, these factors have decreased the power output of his aliens.
But that all changes when he unlocks Atomix; a transformation that reminds him of his idol, All Might. Both Atomix and All Might share lots of similarities with both of them having a stereotypical superhero persona. They are boisterous, enthusiastic, courageous, confident and righteous. When Izuku becomes this new powerful, almighty(no pun intended), he feels good. The timid Izuku is gone and in his place is this new being; Atomix, a powerful being who stands tall, a strong man who laughs in the face of danger, a god who believes he’ll come out on top no matter how bleak the situation is.
This idea takes inspiration from Insomniac’s Spider-Man 2 with Yuri Lowenthal, describing the Symbiote as an addiction. I recommend reading that article where he explains that and watch the game so you can get a better understanding.
