How can Fatwa no. 79061 and the Hadith "Whoever conceals a Muslim's fault in this world, Allah will conceal his fault on the Day of Judgment" be reconciled? And is the suitor obligated to disclose his sins and transgressions to his fiancée?

The Hadith ("Whoever conceals a Muslim's fault...") is a general ruling, while the fatwa mentioned is a specific ruling, and there is no contradiction between them. The general principle is to conceal a Muslim's fault and not mention their shortcomings, as this falls under prohibited backbiting, except in exceptional cases where backbiting is permissible: seeking justice, seeking help to change a wrongdoing, seeking a fatwa, warning Muslims from evil, an open transgressor or innovator, and identifying someone by a nickname if they cannot be identified otherwise. The suitor is not obliged to mention his sins; rather, he must repent and conceal his own faults. However, if he persists in sin, then it is forbidden for him to deceive and mislead the one he intends to marry.