Is the ghusl and prayer performed by the questioner valid, and what is the ruling on her abandoning prayer during the days interspersed with brown and yellow discharges?

The maximum period for post-natal bleeding (nifas (definition)) is forty days. Anything beyond forty days is considered abnormal bleeding (istihadah (definition)). After the forty days, one must perform ghusl (definition) (ritual bath) and pray, performing wudu (ablution) for each prayer, just like a woman with istihadah. As for the prayers missed under the assumption that they were part of the nifas period, it is more cautious to make them up.