Thanks so much Kratos. I called AT&T today and had them turn off VoLTE for my line, as I figured that was the lowest risk, highest return strategy. Took about 15 minutes, had to get to 2nd level support, but they did not give me a hard time or anything. I'm receiving calls on LTE all day since the change with no adverse affects detected.You can simply call AT&T and tell them to turn off HD Voice/Voice over LTE for your line. I wouldn't suggest you flashing a radio for your issue, plus CID would need to match which it may not.
[*]If I end up going RUU, does it end up factory reset / wipe? Yes, RUU will set the full device back to stock, factory configuration.
[*]If I end up going RUU, does it end up relocking bootloader? I've never been S-On & bootloader locked for more than 5-10 minutes, I know for OTA bootloader would need to be locked but for RUU, I think unlocked bootloader will be fine and it shouldn't re-lock your bootloader.
[*]If I end up going RUU, does it end up unrooting? As stated before it will set your device back to full stock, factory settings so yes it will unroot.
[*]My understanding is that S-Off, once achieved is permanent, correct? Yes, you can always go back S-On but you will have to be fully stock or that will result in a brick, so it's recommended that once S-Off to remain S-Off.
[*]If RUU basically takes it to stock, I'm guessing I need to re-unlock, re-root, restore my nandroid backup. Is this the proper gameplan? As I stated I don't think you will need to re-unlock your bootloader as it should be able to do the RUU while unlocked and remain unlocked through the RUU process but you will need to reflash recovery, what nandroid backup do you have, I would recommend a clean install to be honest.
Personally I would recommend to S-Off and SuperCID so you can convert to WWE and get the latest firmware and ROMs as early as possible. Especially as AT&T has started being rather slow with updates and as AT&T is the closest to WWE and most things will work perfectly and when AT&T does update, carrier support will be added and the carrier specific features would work as well. But in the end the decision is up to you.