Is it necessary to repeat the prayers of the past year due to the presence of brown discharge from a C-section wound, which I did not know could be removed until after attempting to rub it off?

You do not need to repeat any of the prayers, because this residue either has no substance originally, so it is not considered an impediment, or it has substance, but its removal is difficult due to its strong adhesion to the body, and this does not affect the validity of purification. Some scholars pardon a minor impediment where it exists.