I don’t think the latchkey kid would fit someone like Starlight, mainly because she’s not a vulnerable character. She’s the most powerful character in the show because of her many odd magic spells.
The latchkey kid approach would work much better with someone like the Cutie Mark Crusaders characters. Babs Seed from One Bad Apple, Diamond Tiara from Crusaders of the lost Mark (which has shown a perfect logical reason why she acted the way she acted), Coco Pommel from Rarity Takes Manehattan… and if you want a more recent example, Misty from Make your Mark. She works for many reasons:
-Misty was properly introduced in G5. The writers took their time to develop her. It wasn’t just 2 episodes with minimal presence.
-Her motivations and conflicts are simplier and more defined. She wants to please Opaline, but she’s clumsy at that, and, while she tries to jinx Sunny and her friends, she also discovers that friendship might not be a bad thing.
-Misty doesn’t have power and she is more vulnerable. She doesn’t even have a cutie mark, she just paints it. The more vulnerable a character is, the easier is to empathize with her as a viewer.
-Most importanty, there are clear stakes which the audience can get behind. After watching the last episode of chapter 2, Misty would have to make a choice: Either still working for Opaline despite the treatment she gets, move to Maretime Bay or another option.
Considering the writing approach of Make your Mark and what we know so far, I have faith in these writers.
And there you go, a recent example in MLP about how to make the audience root for someone.
By the way, if Starlight really cares about Sunburst, why didn’t she write some letters for him instead? Why couldn’t she just ask him what his new place is so she can visit him and send him letters so she can have fun communicating with him? But no, he moved away and that’s where we draw the line.