Absolutely nothing you could have done to safely prevent mosquitoes from transmitting the fowl pox to your flock. That they all get it is inevitable, having them all get it at about the same time is a blessing as once they've recovered, they're immune to it for life. There are two presentations for fowl pox; wet or dry. The wet form is life threatening. It is characterized by mucous in the nostrils, sneezing or coughing, sometimes the affected bird will shake their head as if trying to fling the mucous out of their respiratory tract. The dry form is more of a nuisance than a threat, and you've posted pictures of it, so you know what that looks like
Regardless of which form you're presented with, the best you can do for them is to keep them comfortable, ensure that they're eating healthy to bolster their own natural immune system, and pray. Please don't put apple cider vinegar in their water, it tastes awful and discourages drinking. Right now, they need to remain hydrated, so their water needs to be as attractive as possible for them.
