Hi Primmy! I personally don't get a lot of sun where I live either so I've never tried to sunfade. What color are your marked-up ponies? If you have a few white ones, you might want to consider purchasing a product called Remove-Zit by a company called Twin Pines of Maine. It might be about $20-$25 with shipping, and I do NOT recommend using it on ponies that aren't white- it will fade them, saddly
But I had really great results getting red marker of a G1 Scoops.
That's the only tip I have; hopefully someone else can give you more. Have you tried to match Daffidazey's hair colors with shades on Dolly Hair? Off the top of my head, the blue looks like Water Nixie.
Colorscapeart has a good point about the cost of replacing vs restoring G3s. Still, it always feels so great when you are able to save a pony and make her look (mostly) new again. Good luck with your ponies!