Lice and feather pulling

We have had our 3 chickens for about 3 months now, and they seem to have settled in very nicely. However one of my girls, my blue maran, is causing me some concern.

Initially, one of the other used to be a bit of a bully and peck at her quite a lot. This eventually settled down, but I did notice that the maran seemed to peck at her own feathers quite a bit too. I checked her over and discovered that she had quite a nasty lice infestation. The other two were clear though.

I have been regularly dusting all three chooks with gold label louse powder, as well as adding it to the dust bath and coop, but she still seems to have lice (although nowhere near as bad as they were). Because I am not convinced it is working, I have today bought some Johnsons anti-mite extra spray, which someone recommended to me.

My real concern is that my chicken seems to have plucked a bald patch on her chest over the past few days, as well as a couple of small areas on her wings. I am really worried that the others are going to start pecking her skin, and we are going away next week and leaving a friend here looking after them who knows nothing about chickens. Is there anything else I can try in the next few days to try and get on top of the louse issue? Or could it be something else causing her to pull her chest feathers out?