Johnsons do a handy anti-mite spray for small animals and birds-its pyrethrum based. Works a treat on lice with a few pumps under each wing periodically.
Ivermectin should work (I've used it for a persistent case of scaly leg) but it will only work on lice feeding on your hens at the time-which is why its no good for red mite-plus there's the egg withdrawal. If you have an infestation, treat the whole flock three times at weekly intervals ( with whatever you decide to use) to make sure you get all of them, including any eggs due to hatch. TBH I think DE is completely useless for anything, we have colonies of house dust mite, sheep scab mite and poultry red mite at work and we've tried it-does nothing to them at all.