I think it depends on the episodes and how good these are. For example, I really enjoyed Parental Glideance to the point where I don’t care if it came out too late, even though it should have been an episode from season 4 or 5.
However, Frutter Brutter was not very good to me. It shows Fluttershy’s parents, but the episode isn’t about them, they were a bait, it’s about Fluttershy’s brother, and I really didn’t like it because of him. So, even if you show this episode in season 3, I’d still have the same impression.
If your episode is excellent, I think the “it came out too late” impression wouldn’t matter. If it’s not good enough, then no.
I don’t know, I think you could have many episodes for Starlight and Sunburst in season 5, not just 2 (one for the opening and one for the ending). You could have 2 ponies from the Mane 6 trying to find Starlight. Then they could find her parents, some old friends, some old towns, probably a diary… There’s a lot of different ways to make her an interesting character. Not just ignoring her for 24 episodes.
About season 6, I don’t think the problem was that Starlight got too many episodes. I think she got enough episodes. The problem to me was the narrative approach. Back then I was a bit indifferent, like “ok, what are you gonna do now?”.
Now that you mentioned, Hearth’s Warming Tale is still the episode that I liked the most about Starlight. Keep in mind that was before Every little thing she does, so seeing that episode back then made me think “maybe Starlight would be an interesting character”.