Admitedly for how EQG handled redemptions, even the main show did share those flaws too. Several reformed characters aren't seen after they reform like Diamond Tiara, Silver Spoon, Gilda, Garble, Dr. Caballeron, Chancellor Neighsay, and Ahuizotl. And even with the reformed ones who appear post-redemption, Stygian and Babs Seed only got one appearence afterward (and even then, both hardly do much in those appearences), leaving only Luna, Discord, Starlight, and Trixie as the only ones to frequently appear post-redemption in the main show. So that issue with EQG does have its roots in the main show on some level.
Also, wasn't Luna instantly remorseful for her behavior the moment she was snapped out of her Nightmare Moon mode? Same for Starlight turning good after Twilight reaches out in the season 5 finale. I've seen some criticize Starlight for how everyone like Twilight, the rest of the Mane 6, and even Our Town forgive her pretty quickly (compared to Sunset being a social pariah for most of Rainbow Rocks).
I do agree with you on a lot of points. It annoys me how the main characters befriend these reformed characters, but we never see them interact with them again. Diamond Tiara is a great example. We learn about the pressure she is put under by her abusive mother, and she becomes friends with the CMC…but we never see her interact with them again. Yeah, we get a few scenes with her having fun with Silver Spoon and another filly, but nothing else on top of that. And I still have so many questions regarding Diamond Tiara. Such as, what is her relationship with her mother like now? Spoiled and Filthy Rich are still together, but the idea of Diamond Tiara continuing to live under the same roof as her abusive mother is horrifying. And none of this is addressed.
I wish that we could have seen more of Dr. Cabelleron and Ahuizotl as well. But I understand that the creators of the show just wanted to wrap up the Derring Do arc. That’s a big reason why many of these villains reformed and never seen again. The show just wants to wrap up these story arcs within the show, and move on. And while I understand these choices, as they want to focus on the main story that they are telling, it highlights a problem with the show: too many characters. I love MLP and credit them with my love of reformed villains, but I do feel that they reformed too many characters, and it made it dissatisfying when these characters who found a better way of life were never followed up on again.
However, it’s worth noting that a lot of these aforementioned characters were reoccurring characters throughout the series. In EG, a lot of these characters were introduced in a movie, were revealed as a villian, reforms, and are never seen again afterwards, which almost makes it worse, as we never get to truly familiarize ourselves with these characters that we’ve just gotten to know, whereas we’ve known characters like Garble and Diamond Tiara since season 1.
I don know if I agree with you entirely about Princess Luna and Starlight Glimmer. Any hesitation or lingering resentment that Princess Luna might still have held towards Celestia seemed to disappear upon their reunion (“I’m so sorry! I missed you so much, big sister!”). Also, we didn’t see Princess Luna at all until season 2, so we don’t really have much evidence that proves that Luna struggled to let go of her resentment.
As for Starlight Glimmer, she seemed kind of nervous, or anxious around Twilight initially following her redemption. Twilight and Starlight actually become very close as teacher and student, as well as friends.