Sorry for all your troubles! What you describe may be tropical fowl mites. The dose for Ivermectin injectable is 0.2 mg/kg, which is 0.009 ml per pound. So that is how you want to dose the birds. Weigh them all first to give an accurate dose. Some sources also say 0.4 mg/kg, so you could safely give up to that amount, too.
Then, you will want to clean out your loft VERY well, and dust it all down. Use the stuff you have for dogs to dust it with now.
Then, you will want to clean out your loft VERY well, and dust it all down. Use the stuff you have for dogs to dust it with now.