[T-Mobile Samsung Galaxy S7 Edge Questions & Answer] Synchronicities:
So I've spent a few hours searching XDA to find the right resources on how to flash a different U.S. carrier's GS7 Edge firmware on the T-Mobile branded device. I found out that since all the internal hardware for all U.S. devices are identical, I'd be able to flash any one carrier's firmware on to any carrier's device, provided there are no inconsistencies with the bootloader versions of the firmwares.

The phone is intended for an extended family member and I am able to purchase a T-Mobile GS7 Edge for significantly cheaper than from any other carrier. Because it will be used on AT&T's network, I wanted to make sure I got everything right before I get started. I either want to flash the AT&T version which will allow it to be fully compatible on the network, or the Unlocked version. Apparently, the unlocked version is completely debloated since it's not associated with any carrier and provides better benchmarks, especially battery life, so I'm interested in that.

I've compiled the resources I need in the following:

https://xdaforums.com/att-s7-edge/how-to/firmware-att-s7-edge-g935a-official-t3412312

https://xdaforums.com/att-s7-edge/help/att-t-mobile-bootloader-version-t3473281

https://xdaforums.com/tmobile-s7-edge/help/installed-sm-g935u-sm-g935t-firmware-t3482945

https://xdaforums.com/att-s7-edge/how-to/samsung-sm-935u-firmware-install-guide-t3411451

https://xdaforums.com/att-s7-edge/how-to/firmware-pi2-s7-edge-sm-g935a-10-03-16-t3477246

https://xdaforums.com/verizon-s7-edge/how-to/odin-stock-nougat-firmware-t3570156

https://xdaforums.com/att-s7-edge/how-to/updated-935u-935a-live-links-pi3-t3501686

http://updato.com/firmware-archive-select-model?q=SM-G935U&order=date&dir=desc&exact=1

etc.

One question I have is whether I am able to flash the Unlocked firmware at all. I don't currently have the device in hand so I can't check the baseband version, but my understanding is that the Unlocked version is currently on a lower bootloader version whereas it is v4 for AT&T or T.Mobile. This would mean I would be able unable to flash the Unlocked firmware if the device already has a higher version of the bootloader, right? Conversely, I would be able to do so if the T-Mobile bootloader was on the same version as the Unlocked version (e.g., if the device hasn't had an update to its bootloader).. The same issue would arise if I wanted to go back to a carrier firmware. Where can I find a list of T-Mobile firmwares and their bootloader versions so that I can check? Threads I've searched are saying I can't go to 935U from 935A/T because of mismatching bootloaders but others are saying I can go back to 935A/T on Nougat (same bootloader versions?) so clarification would be greatly appreciated.

Edit: Just discovered the bootloader version can be determined by looking at the 9th character in the baseband version. Pretty simple!

Another question I have is whether flashing the AT&T firmware will give me VoLTE, Wifi Calling, tethering, etc. on AT&T's network. I've established that if I flashed the Unlocked firmware, I'll lose these functions (unless I root the phone, but that's a no-go since I need Samsung Pay intact). Just wanted to make sure that I would retain these features if I flashed the AT&T firmware on the T-Mobile device. Also, would I receive all OTAs pushed from AT&T while I'm on their firmware, even though it was a T-Mobile device? Or would I have to manually update it somehow?

Many thanks in advance.