Hi Blinkers, we've had them as well. Colin should recover fine now that they are off him. To get him in that state there will be tens of thousands in the coop feeding on him at night. The powder is only a discouragement and won't keep them at bay for long though; the little buggers know its a feed or die situation. Even in the most extreme infestations we've never found them on the birds during the day, so yours must be bad.
Plastic coops are easy to treat. Completely dismantle and spray with Smite and/ or Nettex.
Wood is a whole different problem. The mite hide in all the joints, particularly the top of the groove in the tongue and groove boarding joints. Spray won't reach them all so it is a constant battle to limit the numbers. Even a steamer won't get them all, but if the wood is dry and the gaps large it will get into the grooves. The only solution we have found that works is to completely dismantle the coop and treat all the sides upside down, so that the grooves are reached, with a 50:50 mix of the old creosote and paraffin. Neat creocote (the safer new stuff) is less effective but better than chemical sprays, which don't shift the eggs. We did try a biocide called Elector (attacks the nervous system) which worked the first year, but by the second year they had evolved immunity, as they do with the chemicals anyway. Re-apply 50:50 every 12 months but I doubt you need to dismantle again, don't know as this is our first year with this treatment.