Sorry to hear the sad news about Coho. I used a spot on treatment last year for treating lice when I had a bad outbreak. The one I used is called Xeno but I don't know if it's available in the US. It was expensive but using DE wasn't getting on top of the problem. Wild birds mix with my flock all the time so I guess they bring the problem in.
From what I have read and researched, DE isn't worth the money. Here in France I use a dusting powder called Rhodeo (based on pyrethrin and geraniol extracts), and it works amazingly well on lice.
I know that Nambroth did everything she could for Coho, but in the end there is nothing you can do against Marek's . It is impossible to prevent contact between wild birds and a domestic flock, no matter how hard you try.