I like the usage of color here, and those are good TL notes. While I don't necessarily agree that watashi is definitely feminine (I was taught in 日本語 language classes that this is gender-neutral and perfectly acceptable to use by both sexes), I suppose it got a slight feminine connotation to it in practice, because in media, men typically won't use it in a casual setting with other men of similar social status (e.g. at a drinking party) or in situations where verbal assertion of masculinity is somehow necessary. Atashi, however is unambiguously feminine. I don't know if they used it here, but I can easily imagine Akira's female bullies using atashi to assert their femininity, rubbing the salt into her wound.
In my view, Akira's mother is even harsher by not allowing Akira to use even a generic, gender-neutral pronoun. She forces her daughter to unambiguously speak like a male and be a replacement for her father.