While I agree with your points that losing the one dear friend she had to a cutie mark can impact her psyche, however I would have to agree with Clever clover that despite the cause for her beliefs, she still behaved abhorrently as this was the same person who had a nefarious scheme with the mane 6 cutie marks that the finale never made mention of for what her original goal was (though I can accept it may have been trashed, it still doesn't excuse it for not mentioning what she had planned), also I agree with him that this was the same pony who tried to blast Filly Rainbow Dash without a 2nd thought just to spite Twilight in her efforts to stop her. That is not the actions of a pony who is wholly misguided but one who has clear moral issues and has little problems to enact questionable actions just to get what she wants. It was a good thing she wasn't so heartless as to allow Equestria to be ruined for her own ends unlike Radiant Hope...
Thanks to your excellent points however, I now believe my biggest issue with Starlight Glimmer's redemption isn't how seemingly quick it was but rather how her redemption white washed her actions she had committed against the mane 6 just to satisfy her needs for her cause. If the next season at least tries not to show her completely reformed and there is still plenty of room for her to grow as a character, than I will have to agree near whole heartily with your view on Starlight Glimmer. Until than, I will agree that she has one of the better redemptions right now, but that opinion can easily change in the future if she is poorly executed after.