Perhaps in his youth he had indeed been infatuated with and idolized Voldemort, but in later days, he might have grown disillusioned by certain actions of Voldemort and thus chose to switch this Horcrux, leading to his death.
Yes, Quinlan was certain that Regulus Black must be dead.
To retrieve the locket, one inevitably had to go through the ordeal they had just experienced: crossing the lake of corpses, drinking the potion to the last drop, suffering from agonizing thirst, and finally crawling wretchedly onto the shore, only to be dragged by Inferi into the bone-strewn depths of the lake—ultimately becoming one of the Inferi guarding this Horcrux.
Chapter 163