[Moto G5 Plus Questions & Answers] echo92:
I don't own a G5 Plus, so up to you how much you take on board (I have a G4 Plus) - as always, please double-check anything you're not sure of. Stock ROM firmware is not to be trifled with...

1)Either way, to use OTA updates on the stock Motorola ROM, you'd have to re-flash back to stock (full stock, not TWRP flashables). You do not have to re-lock the bootloader, OTA updates can install with an unlocked bootloader. The key rule appears to be at least un-modified (read-only) stock system, stock OEM, stock recovery, stock logo and stock boot.img (kernel) partitions that are from all the same stock patch level and match the patch level of your bootloader (mixing and matching patch levels, e.g. older stock firmware with a newer bootloader is a really bad, brick causing, idea...). If you swiped to let TWRP make modifications, then you'll likely have modified the /system partition and require re-flashing/restoring an unmodified /system partition. Easiest way to restore these all to stock would be a firmware flash.

a)Leaving your bootloader unlocked allows you to root your device later, plus easier for troubleshooting. Only requires re-flashing stock firmware (which can leave your data intact, though back up as always).

b)Re-locking your bootloader is more complex, plus involves erasing your data. Cleaner, but does not restore your warranty. However, depending on your service centre and US consumer laws, service centres may take your device in for repairs depending on the fault.

c)Either path would require you to re-flash the same/newer stock ROM onto your device to successfully accept and install OTA updates. As you're on the Jan 2017 NPN25.137-33 update, that is the original update for US/EU devices (though as soon as you update, you cannot roll back to this build without potential issues). If you're worried about hard bricking, then re-locking will force you to only accept updates. Downgrading your stock firmware is only risky if you take OTA updates, so either re-lock your bootloader or do not downgrade your device stock firmware. That means only flashing NPN25.137-33 or later firmware, and then staying in step with your device firmware as you update. Staying in lock step can be difficult due to the sparsity of certain stock ROMs (Motorola is not keen on releasing stock ROMs, so generally it's been up to us to 'acquire' said stock ROMs.) Hence, be careful what you flash.

d)Either method would remove root, TWRP and magisk, though you could re-apply them once you've updated.

2)NPNS.... updates are for G5 Plus devices, the S denoting a security update (though Motorola's update naming system is a little inconsistent). G5S Plus devices appear to be running updates starting with NPS, G5 and G5S are NPP.

NPNS25.137-33-5 should be the next update in line for you, depending on your network (other devices on other networks do appear to get different updates, especially Verizon). The OTA servers should be suggesting the correct OTA build anyway!

3)Should be able to use the guides, just ensure you use the correct stock ROM. If you're on NPN25.137-33, here's a link for the stock ROM of the same build: https://mirrors.lolinet.com/firmware/moto/potter/official/RETAIL/

Also, you may wish to look into using this stock ROM (NPNS25.137-33-5, May 2017 security patch, should be the same as the update you're prompted to receive) to update and return to stock in one go: https://androidfilehost.com/?fid=961840155545594421

Either way, if you just want to revert back to stock, and wish to keep your data, then do not re-lock your bootloader and omit any fastboot commands with erase userdata (erase cache is okay) - such as the following fastboot commands you can try. Of course, back up your data as always just in case. Ensure also that you flash the same build as what is currently on your device - other builds may work, but render OTA updates inoperable unless you revert to the intended build for your region (but this process may damage your device and Motorola may ask you to pay for any damage caused in this regard).

Please verify these before using, check the number of sparse chunks matches the number in the stock ROM, and that these commands will not re-lock your bootloader. If you get pre-flash validation errors or security downgrade errors, double check what stock ROM you have present on your device and what stock ROM you're flashing, as these errors appear if trying to flash an older stock ROM (which as I've mentioned above is NOT recommended):

fastboot oem fb_mode_set
fastboot flash partition gpt.bin
fastboot flash bootloader bootloader.img
fastboot flash logo logo.bin
fastboot flash boot boot.img
fastboot flash recovery recovery.img
fastboot flash dsp adspso.bin
fastboot flash oem oem.img
fastboot flash system system.img_sparsechunk.0
fastboot flash system system.img_sparsechunk.1
fastboot flash system system.img_sparsechunk.2
fastboot flash system system.img_sparsechunk.3
fastboot flash system system.img_sparsechunk.4
fastboot flash modem NON-HLOS.bin
fastboot erase modemst1
fastboot erase modemst2
fastboot flash fsg fsg.mbn
fastboot erase cache
fastboot oem fb_mode_clear
fastboot reboot

Good luck whatever you decide.