Lots of questions here but I can answer about overpatches. Dexcom will provide them for free, there's an order form on their website. I haven't tried those yet, I use some from Amazon and these in particular are nice for restarting sensors because the adhesive is in an striped pattern, which makes it easier to take off without removing the sensor. Just make sure to remove it form the inside out.

I live in a swampy city and love these overpatches! They last 7 or 8 days so usually I'll wait a couple days to put them on. The way they make the adhesive makes it easy to pull off from the inside and replace the overpatch without taking off the whole sensor if it doesn't last

I also get my overpatches on Amazon. I live in the hot and humid part of Texas and have never had problems with itching, just with peeling from wear. The brand I use is fixic