Must one re-perform the prayers and fast the day of make-up (qada'), if it becomes apparent that the blood discharged after the customary period was not istihadah (post-menstrual bleeding) but rather menses, especially since the questioner suffers from purity-related obsessive-compulsive disorder (waswas al-tahara)? And is orange-colored blood considered menses?

What you mentioned, that scholars do not consider orange discharge as menstruation, is incorrect. You should disregard and ignore these whispers [waswas]. If you are certain that you prayed a make-up prayer during your menstrual period, then those prayers are not valid. If you left the ghusl (ritual bath) after the cessation of menstruation, contenting yourself with the first ghusl before it, then there is a difference of opinion regarding the obligation of repeating [the prayers]. A person afflicted with waswas is permitted to adopt the easier opinion to alleviate hardship.