Must I make up the two days I fasted, given that I saw a brown discharge during them? And is the woman who experienced a wet dream and did not perform ghusl sinful for not being able to distinguish between semen, vaginal discharge, and pre-seminal fluid?

Fasting on those two days is valid, and there is no need to repeat them. The default is the continuation of purity until certainty of its cessation is established. Any brownish discharge after seeing the signs of purity is not considered menstruation unless it occurs during the accustomed period. As for someone who experiences a wet dream and doubts the emission of semen, ghusl (definition) (ritual bath) is not obligatory until she is certain of the emission of that which necessitates it. If she doubts the nature of the discharge, she has a choice and can assign to it the ruling she wishes. We warn against giving in to obsessive doubts (waswas).