When I fostered kittens who had heavy lice infestations, I was also working at a very reputable vet clinic. We used a tiny amount of frontline diluted with saline to help get the lice off of the 5 day old kittens.

This was an extreme case however; these kittens were being sucked dry by the HUGE quantity of lice on them. I would say keep doing what you're doing, it sounds like it's working.