unbaked, It sounds like you've done a fine job. Remember, just dust it with the permethrin, don't overdo it. In 10 days, you will do it again. The first application kills the bugs, the second kills the nits that have just hatched. By doing it the second time in 10 days, the nits don't reach adulthood, and can't lay eggs.
You mentioned treating the coop, but did you dust the birds? That's important in breaking the cycle.
When cleaning, disinfecting, and treating for things in a coop, it is wise to start at the top, and work down. Always remove the waterer(s), and feeder(s), then wash, rinse, and replace them after the coop is done.
Mites happen. Wild birds, squirrels, field mice, rats, are just a few examples of how mites are easily spread. They are typically easy to get rid of, just keep an eye on the feather condition of your birds. If the feathers look a little "chewed on", or ratty, and they are not in molt, it's time to treat for mites. I went a couple years with no problems, had to treat a couple times about 3 moths apart, then went another year, and a half without incident. There is no set schedule, so don't just automatically treat for them.
I make soapy water with Dawn blue dish liquid, just like dishwater for handwashing dishes. I put that in a sprayer, and spray on the sand until the top layer is damp, in my coop about every 8 weeks.
There really is no vaccine to protect against respiratory illness, but healthy chickens don't typically have a major problem with it.
Getting a couple more birds is fine, but not yet. Make sure your coop is free of mites, and that the remaining birds are in good health. It would be a shame to get healthy birds, and expose them to something that would jeopardize their health. When you are sure everything is fine, get the new birds, but be sure to QUARANTINE them. Don't just put the new ones in with your existing flock. I can not stress this enough. Keep the new ones away from your existing flock for 3 weeks. Far too many have regretted not doing this. They lose some birds, get everything fine, then bring in a few new birds that look fine, but discover in a week that they are not healthy, and then contaminate their healthy existing flock too. Don't make that mistake.