I don't have Peacocks, and know nothing about them, but I'm sure ectoparasite treatment in poultry can be extrapolated to use in Peafowl.
For poultry, I use a Permethrin-Based livestock spray, like this one-
https://www.tractorsupply.com/tsc/product/gordons-permethrin-10-livestock-and-premise-spray-32-oz
Per the directions I dilute the mixture down accordingly into a one-gallon sprayer, shake well, and spray each bird all over, focusing under the vent and wings. Do this to all birds, clean the coop out well, and spray the coop thoroughly, and repeat the process in seven/ten days since this won't kill the eggs. If there are lots of knits (eggs) on some birds, sometimes I'll directly clip them off and properly dispose of them.
Sevin dust will work too, I just find the whole process a lot messy, and time-consuming compared to the spray, but to each his own.
If you're using sevin I'd see no reason why you would want to use Ivermectin. I wouldn't dose it in water since it's not water-soluble if you do decide to use it.