Let's not leave out the fact TMUS loves to give out "free lines" every blue moon. I have two fully activated SIM cards with unlimited everything + HD Video + Hotspot just sitting in a drawer not being used.
No it's real VoNR. It sets up the equivalent of a dedicated EUTRA session (I forget what it's called in NR) for it with the proper bearer. NSG isn't really able to decode a lot of the VoNR signaling to get codec details and such yet. There are likely a lot of issues with it still. It's probably not supposed to be public yet, and it's a fluke that you can access it by forcing SA. It probably won't launch more widely until the x60 comes out, which is the first modem that's supposed to support it. Regarding the SIM swap, I doubt it. In my experience, changing the SIM resets all locking back to none/default. But you can certainly try.