[LG V30 Questions & Answers] ChazzMatt:
Wow you are asking a lot of questions, and that's a lot of typing to answer. ;)

I'm not a big fan of Sprint. Most people try to get away from them, not run towards them. ;) That's how Frankenstein method was perfected, everyone with "cheap" locked-down LS998 trying carrier unlock their phones. Or people with AT&T H931 who didn't have AT&T and wanted updates. Figured out how to flash them with carrier unlocked firmware like US998, AS998, H933 -- or even flash to Verizon VS996 (which is not carrier locked to Verizon). However, then with WTF bootloader unlock method, you needed fastboot flash commands -- and of ALL the North American variants, only US998 has that. But originally

Frankenstein

was basically just to carrier unlock AT&T H931 and Sprint US998 phones...

You may be a world of trouble with Sprint. Your saving grace may be you can activate on Essential, then swap the SIM card to your H931/US998/LS998 phone once you have bootloader unlocked, rooted and flashed the LS998 zip file to "convert" it to LS998.

There are others who do use US998 firmware, but have merely flashed the Sprint modems to get all the Sprint bands. And they populate the Sprint APN. There's a big Sprint thread that details all that. But ALL of them had to first activate their LS998 prior to doing anything with bootloader unlock, TWRP, etc. One guy managed to do it after the fact but it took three pages of help. These are people with original LS998. No one that I am aware of has done it with a model that was not originally LS998.

But I'll repeat -- Your saving grace may be you can activate on Essential, then swap the SIM card to your H931/US998/LS998 phone once you have bootloader unlocked, rooted and flashed the LS998 zip file to "convert" it to LS998. Those other people didn't have another phone to activate (if I understand Sprint thinking, which I may not).

1. How are the Sprint bands on the US998 firmware - aren't there some missing?

Yes, there are two bands missing. Depending on where you live that may not be important. But you can flash Sprint modem, while keeping US998 firmware. Or you can fully flash to LS998 zip file to convert to LS998 while keeping TWRP and root.

4. Especially since eventually, when I go to sell it when I upgrade again, I'll probably want to flash it back to the H931 firmware with no problems from Sprint claiming lordship over it.

AT&T, like Sprint, does not publicly release their KDZ firmware. While you can flash back to H931 stock "ROM" via TWRP, then you lose fastboot flash commands -- so you cannot lock back bootloader and how do you get rid of TWRP? When you even go to sell it, best to sell it as US998. You can relock bootloader and flash full stock KDZ. There's TONS of former LS998 V30+ being sold on eBay as US998. Already Frankensteined by the sellers, and they are carrier unlocked.

3. Also, I didn't back up my modemst1/modemst2 files before I flashed over from H931. Didn't think it mattered since I could just flash back to the H931 someday, but are those files unique to my particular device/IMEI?

Doesn't matter. You can always flash to US998 to get those US998 bands, you can

flash back to H931 via TWRP with stock ROM

if you want those bands back. Flashing to LS998 ZIP file will give you those bands. The one thing you probably can't do after you start is completely restore the H931 to full stock with bootloader locked back. But that's no big deal, for selling on eBay. Just sell as US998. It's more versatile anyway and has fastboot flash commands. I'm an AT&T customer for 7 years and I've NEVER owned an AT&T branded phone.

2. If you were to activate your V30 on Sprint, and then later on flashed a different firmware, would they see it as a different device altogether, and force you to pay the activation fee in order to get it back on again? Or is it just tied to the IMEI, no matter what firmware you're on (say you flashed a stock LS998 to the unlocked US998 using the Frankenstein method)

The only people I can answer for is Sprint customers who had original LS998, activated them -- converted to US998, unlocked bootloader, installed TWRP and rooted, and then still continued to use their devices on Sprint. To still use devices on Sprint, some used TWRP-flashable zip file to convert back to LS998 while still keeping TWRP and root. Others just kept US998 firmware but flashed Sprint modems. Others have flashed AOSP custom ROMs, but had to use Sprint APN and maybe Sprint modem.

I don't want to discourage you. You may be able to get away with this, but only because your ace is the Essential phone. Activate that first. Then, do all the stuff for the H931/US998/LS998 conversion (including bootloader unlock, root). Put the SIM card in, then see if it works. If it doesn't, then what people have found is TELEPHONE help is the best. If you go to a store, they will immediately see you do not have a Sprint LS998. Whereas over the telephone, you can tell them you have LS998, the firmware version, etc, and it's often off-shore service who may not care it's not really Sprint hardware, if the firmware matches....

Good luck!