I would def repeat the application since the mites go through a laying cycle and you might have some hatching eggs starting the process over. As for the young new hens: I would just spray a little under the wings and butt area and keep them quarantined for a couple of weeks. That’s a good practice in general with any birds coming in, mites or not. When I raised my neighbors chicks they came with mites and lice just being a few days old and I took a cotton swab and dabbed the elector mixture under their wings. Bugs were gone in a couple of days. It is a safe product.
If you continue seeing mites even with multiple applications of elector you can message me and we can talk about dosing your chickens with Ivermectin. It’s a very controversial thing in this forum. I prefer to talk about that in private. I have done extensive research in English and German and I have done it myself to our rescue chickens and had great success.
I wish you a mite free coop! Hang in there. At least you know it won’t ever be this bad again since you will have a doctor in mites after this one