Looks like
@Eggcessive has covered it all.
She's posted dosing on #26 and #29
https://www.backyardchickens.com/threads/rooster-seems-off.1412301/post-23190040
https://www.backyardchickens.com/threads/rooster-seems-off.1412301/post-23190077
The Ivermectin *should* systemically treat the SLM, but I would apply vaseline to the legs to help heal the scales and smother any mites that may remain.
I prefer vaseline or most other ointments/oils for apply to legs/feet, face and combs - I've found that some oils can make skin peel, especially on the face.
I think I remember seeing the video a few days ago. I agree, I would treat for crop issues. I'm sure that
@Eggcessive provided that info as well, but this is the info I use for treating impacted and/or sour crop. Keep in mind that a crop can be impacted and sour at the same time. A blockage can also be further down proventriculous, gizzard or digestive tract - so sometimes there may be an impaction and the crop won't "feel hard" because that's not where the blockage is located.
https://www.backyardchickens.com/ar...w-to-know-which-one-youre-dealing-with.73607/
He may have other issues going on - sometimes all we can do is try to treat symptoms and see if they get better. If you lose them, then performing your own home necropsy or sending the body to your state lab can a lot of times give more information.