How can a worshiper supplicate for all deceased Muslims to receive mercy with certainty of a response, despite the hadiths indicating the existence of 72 sects in Hellfire from the Prophet's ﷺ nation?

Praying for mercy and forgiveness for all Muslims is a good thing and there is no harm in it. The prophets did it, Allah commanded His prophet to do it, and the Prophet (peace be upon him) encouraged it. The ultimate destination of those whom Allah decrees to enter Hell to be punished for their sins is Paradise, by Allah's mercy.