I think my issue remains that I don't like to be told how to feel about a character and a situation. It feels like the author and the manga wants me to understand and be prepared to forgive Shiho, and I don't feel either of those things. We've all read a million stories with exactly these type of scenarios where the characters don't act like huge jackasses. I'm sure many of us have also been in Shiho's position of knowing the person we like likes someone else and haven't turned into raging assholes about it.
Put another way, if Shiho was a guy, then she'd be an incel, a "nice guy", or just a plain bully, and I don't think anyone would want her to have a redemption arc.