I’m sorry to hear two of his buddies are also sick that means your treating 3 with this one being the worse I hope what your doing gets them all through it. With all your doing it shows how much they mean to you. Thanks for updating.
I'd just do the nebulizer. In the link shagbarkbantams he does suggest using 1/8tea. to their buckets to keep down the slime but nothing is said about using it by mouth.
@Jenbirdee
As for the Oxine ah I used it as a mist shooting it over their heads so it fell onto their faces.
I just buy the spray bottles at Lowes or online mi9x the Oxine in a gallon container and then pour into the spray bottle. I just bought...
if you treating an animal [bird] you do not use the activator i have used Oxine ah for years to spray down my roosts and bedding in the coop and have misted my birds with it.
I am very sorry your Muscovy is ill.
You can buy meds here https://allbirdproducts.com/ but unless you know what your dealing with I am not sure you would know what to buy unless you just want more of the same antibiotic to finish off what you started.
Also consider what @Jenbirdee has shared...