Is a person considered sinful if they forget a raka'ah or washing the face three times in wudu' due to ignoring intrusive thoughts (waswas)? And is the thick fluid that exits when thinking of someone considered semen (maniyy) necessitating ghusl and making up prayers, or is it natural secretions?

The treatment for obsessions is to ignore and disregard them. As for ablution (wudu'), it is not obligatory to wash the limbs three times; washing once is sufficient, and washing three times is a sunnah (recommended practice). Regarding discharges, if you are certain it is semen (mani), then ritual bathing (ghusl) is obligatory. If you are certain it is ordinary discharge, then only ablution (wudu') is obligatory. Ghusl is not required unless there is absolute certainty that it is semen. If you are unsure whether the discharge is semen or something else, you may choose and assign it the ruling of one of the uncertain things.