Are these the latest no slip cinch buckles? Those are the ones I have and they grip really
I use two hands but adjust the webbing without much trouble. To loosen, separate the cinch buckles so that there is a gap of an inch or so. I slip a finger or two between the buckles and then pull the webbing through with my other hand.
To do it one handed, maybe you could put a spacer such as a small branch between the cinch buckles.