In a warm, dry climate such as yours, I don't doubt that you've seen minimal corrosion damage.
Why do you think I moved here?
Cars may not rust, but the heat and UV is hell on interiors though.
leaving the paint on the outside edge of the new hole does--gives more protection.
Sure. But I've never seen a hole saw remove the paint beyond the o-ring.
Spend any length of time in New England, and you'll know what I mean.
My prime motivation for moving here - grew up in WI, then ME and MA for a decade.
that o-ring insulated the mount from the roof metal--unless it is overtightened.
I don't see how you could tighten one without contacting the roof. The point of removing the paint is so you didn't have to rely on the underside teeth because you wouldn't normally be able to, or want to gain access there to scrape the paint. If you have endless time to painstakingly massage the process you can make just about any method work. With a hole saw, poke the hole, run the line, tighten the ring and get on with your life.
I can't say how many NMO's I've done this way but it's certainly many hundreds, possibly thousands. My pickup truck I put its' first NMO in around 1986 back when I lived in Maine is still working perfectly, leak and corrosion free to this day (I've added 5 more to it over the years). The last thing you want when doing this professionally is to have an installation come back because it's not working right, especially when it was your technique or error that causes it. You don't have the time, nor can risk damaging a headliner or trim pieces dorking around with a punch or scraping paint. Especially since you don't have to if you use the tool made for the purpose.
NMO's have stood the test of time because they don't leak, don't cause corrosion and can be installed without access to the back surface. If it required extraordinary measures to install or maintain integrity, they wouldn't have been as widely adopted as they have. The mount and tool to go with it makes this an easy and simple process. Why make it any harder for one's self?