By setting the breakup between middle-school and high school, the author specifically put that “Nene could have gone elsewhere” option on the table.
Yeah but we know that Nene still love Amane so i think she couldn't bring herself to leave Amane all of a sudden and come with that twisted plan.
I've been saying that for some time now, yes. Though it's rather less of a "twisted plan" than "inability to either compromise or choose between two directly contradictory impulses" (ie. her love and hate/envy for Amane) and duly kind of half-assing the whole thing at the other party's expense. Or as she puts it herself, "she has to hate me" - because however chronically mixed up her feelings about Amane were she still balked at going the full mile and conclusively breaking things off for good, ergo she had to try and push her into doing that instead.
People do the damnedest things to cope with their weaknesses and often enough at somebody else's expense.