Both to treat for existing lice and as preventative for internals.
It's all in that thread I linked.
I realized I should have read that a while after I posted. I just wasn't thinking.
The way I read that you found it effective on an active case of lice the first time you tried it but had what was possibly a re-infestation of mites shortly after a ten day treatment so maybe not worth much as a long-term preventative. Makes sense.
I can see why it would not be effective on scaly leg mites. Their life cycle is about twice as long as a ten day treatment plus those mites do not suck blood. They eat other stuff under the scales.
Knowing you I'm sure you did a lot more research in it that my quick look. What I read is that it was approved at that time for certified organic use on crops and externally on animals. Its primary use seems to be on fruit trees and as a hoof treatment on various animals (dangerous on sheep though) but other uses are allowed.
It is also allowed to be fed to correct nutritional deficiencies. I did into see anything on feeding it to control internal or external parasites so nothing in withdrawal times, if any. That might be a dosage issue. It’s highly toxic to some marine animals and over time could build up to high levels in the soil since the copper doesn’t leach away well so disposal is important.
To me it doesn’t sound that dangerous but like any chemical dosage is important.