But there isn’t anything “in Qurirell”. That was a form of Voldemort that existed after his death (when he’d already split his soul into pieces).

What I’m saying here, is Voldemort isn’t human there or when he gets his body back, meaning (I think) no soul. That’s the whole point of the soul splitting. Whatever that form of him was, I don’t think it contained any pieces of his soul, so, the Dementors wouldn’t have any effect on him.

I also think, considering how rare knowledge of the Horcruxes were, if it were possible to slow Voldemort down with a Dementor, Dumbledore surely would’ve tried or passed the info along to Harry.

Destroying the horocruxes was the only path to stopping Voldemort.