I have three of her now, but the first one that I've got is in even worse shape then yours. I did plan on trying to rehair that one till I found another one in better shape.
I did plan to also ask the forum for help as well as try and use this tutorial
https://www.youtube.com/watch?v=BYElWcNtcMo but I'm unsure if that would be needed in this case.
After looking at your pictures I don't actually think that you need to do anything with the mane. I'm pretty sure it's the original hair length for both mane and tail.
The empty space can be filled a bit by lightly moving strands of hairs.
I'm unsure if my explanation makes sense but try lightly moving hair with your fingers it does seem to help a bit (tired to show on a photo)
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loginThe hairs should move in-between "teeth" and stay there. But even without it I think that the mane will look fine after washing.
The bigger issue is the tail, I'm unsure if you have this piece but on pictures you're missing the "bead" piece that holds the tail.
I included some pictures (I took out the leftover hair from my busted Light Heart to see how the everything looked like inside).
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loginIf you don't have the bead you could probably try and use a replacement or a small hair tie. Without it you may loose some hairs when washing your pony.
Overall I think she just needs a good wash (and help with her tail) and she should be fine.