I'm glad you were able to see the vet, I'm sorry that they really didn't find much.
I would assume the vet gave her Ivermectin for the lice so that *should* take care of them.
I would watch to see if she's dust bathing regularly and keep those areas raked, adding the ash and DE wouldn't hurt. As for putting extra DE on her, I wouldn't but if you do, wear a mask since it's so dusty and an irritant.
A fecal float could tell you if worms may be an issue, but I wonder if the vet did one?
If her poop is normal when she's not drinking a lot, then that's good. Drinking all that extra water is what's making her poop runny.