I lean toward the “Igarashi arc is not over” theory, if only because otherwise her character makes no sense, and this author has not seemed to be slipshod or careless in story and character construction.
The whole arc consists of the following elements:
- Igarashi’s flirtatious behavior with Chidori, and Chidori’s reaction
- Chidori’s confrontation with Igarashi about the boyfriend and their subsequent estrangement
- Igarashi’s “not just friends” Valentines chocolate and Chidori’s rejection of it
- Igarashi’s return in the recent chapters
All of these are consistent with her “I was just joking around/two girls can’t really date/just checking in on my kouhai” gambit except for the Valentines chocolates.
Either she meant those chocolates (or more precisely, the note that came with them) sincerely, which means her “only joking/playing around” excuse is just bullshit, or they were a malicious attempt to continue to string Chidori along after the confrontation about the boyfriend.
If Igarashi were to return, perhaps something might be revealed that could theoretically make me more sympathetic to her, but as it stands: fuck her. If you actually care about someone, you don’t go out of your way to make them feel bad.
Besides, comparing her to Nanoha the Great, the only real question is if Igarashi is a malicious piece of shit or a pathetic one.