Oh, good grief! You're having a time of it
. I almost want to say that as long as you continue checking Ellie and she doesn't have any, I wouldn't treat her (again). If the crate is the issue, and I'm betting you're right about that!, maybe you could treat it with Frontline (they have it in a spray), or even buy one of those sprays they sell to treat your home. I of course would do it outside on the porch, or the driveway, or whereever you're able to--I know it's cold out there, so you may not have many options (maybe in the garage even?).....maybe that would have the effect you're looking for BUT also keep Ellie away from those chemicals. I bet the big dog is scratching from bites just like you said....and the eggs that are on it should be dead if you treated him with Frontline too, so that shouldn't be a concern. Comb his neck, the base of his ears, and the base of his tail to check for any new ones since that seems to be their favorite place to congregate, and do the same for Ellie, but I think it will all be over very soon!
I'm not sure how boric acid works, and I'm not sure about it's safety either....that's why I suggested the D.E. I've heard BA works well, and since you've already used it, I wouldn't think you'd need to do anything else, but maybe have the D.E. on hand for next time (which we'll hope you never have to use anyway!!)---that way you'll be able to use something that isn't a chemical.