I don't think this line of analysis works for determining that a work is sexist. At least, it's not sexist in a problematic way (i.e. we need to get rid of it, or at least be aware of the sexism when reading it), it's sexist because the world we live in is sexist and it's practically impossible to write anythong non-sexist. Does HPMOR do anything to advance the condition of women? No, but neither does it do anything to adress racism, ableism, homophobia and plenty of other societal issues. That's not why the book was written. On the other hand, it has two strong female characters that have agency, and no female "characters" that are boobs on a stick, or a reward for the hero, or anything of the sort. Remember that EY was working from canon and could not exactly add plenty of important characters unheard of in the book. He already promoted Daphne, Tracey and (to a lesser extent) Susan compared to their role in the series (although I guess that's partially to make up for the lack of Ginny or Luna in Harry's first year). If your strongest argument to say a work is sexist in a problematic way is "a secondary female character dies without accomplishing anything", then I feel that's too strong a criteria. Hell, even in "Blue is the warmest color" (the book, not the movie, which was admittedly quite sexist in its direction), the main character dies a stupid death without ever having accomplished anything, and you'd be hard pressed to find that book sexist. In Worm, an important secondary female character is killed in an anti-climatic way by a villain we had almost never heard of until the chapter where he kills her. Is Worm sexist? I don't think so.