He (not misgendering) choose to change his sex to show how stupid that law is in bis eyes and to demand to be locked up in a female prison. His change got revoked. Also, he is a fash, always has been.
Time travel is mostly used as a deus ex machina to solve a problem a writer has written themselves in. Like in HP, the time travel bit is only for students to learn more, not to erase attacks by Voldemort, because reasons. In other words, they could have stopped the death if Harrys parents, but did not want to, because reasons.
I get that if used well and as a consistent plot device, it can work. But in HP, its simply a one-oft drama device. Its boring and lazy writing.
Edit, ah, I see
My Immortal is popularly regarded as one of the worst works of fan fiction ever written
Irgendwas, irgendwas DIE GRÜNEN