I think it's a lot simpler than that. I think the WW are hanging back because they know if they die, then all the Wights under their control will fall as well, and seeing as how their plan is to basically tidal wave Winterfell, having them out there on the field wouldn't make much difference. Especially since they know there are dragonglass and valyrian steel weapons in the mix.
I don't think they're literally controlling each wight, they can just more or less control their general direction.
I don't think they're literally controlling each wight, they can just more or less control their general direction.