Sounds like she has polished up perfectly for you
Dreamcrystal's story of her Baby Moondancer is a perfect case in point of this treatment. You can, as you have, bleach out the staining that are the result of aging (age marks). However, unfortunately you are just removing that initial staining and over time it will come back as the plastic continues to degrade. Doesnt mean she doesnt look super pretty now...or that you cant treat her again in the future when it comes back. Congrats on such an adorable pony.