General RUU/ZIP Constraints Question - Android Q&A, Help & Troubleshooting

Hi!
I've been trying custom ROMs on my old phone and want to revert to stock. Despite this concerning a very old phone, I think my question is probably pretty universal, so here goes.
I have a HTC One (M7). I've tried lots of different custom ROMs and now I want to revert to stock. I finally did this using Guru Restore (elsewhere on this forum). However, with flags set for TAMPERED and RELOCKED and S=ON, I am stuck at the most ancient version of the OS. OTA updates wor't work, probably because of the flags. No way to get S=OFF since the options are all no go (revone won't work with this bootloader and I can't fork out $25 for Sunshine). So to be simple and clear, I am 100% stuck with these flags now. Also to make it even trickier, I have a pretty rare/obscure cid in ORANG001. So what are my options for being stock but not the oldest software imaginable?
RUUs: I assume they are the .exe files, since the second 'u' stands for utility. None will work on my phone, either because of the flags, or because none match my obscure cid. So the .exe's are out. Period. Question 1: Unless... does a RUU being 'unsigned' mean it should load anyway?
.zip files: My understanding is that these are the same code as the RUUs above(?), but I'd install them with (eg) TWRP rather than via Windows. So Question 2: If I install a .zip RUU with TWRP, will it ignore a mismatched cid and succeeed anyway? Question 3: I guess a stock RUU installed with TWRP requires no Gapps?
Nandroids: Same question really. Question 4. If I try to restore from a Nandroid that came from a device with a different cid, will it fail, or will it succeed?
Anything else that might help me get a somewhat more contemporary version of Android and Sense onto the phone?
Thank you for your patience and advice & guidence - in anticipation!
Cheers.
(bootloader) version: 0.5
(bootloader) version-bootloader: 1.61.0000
(bootloader) version-baseband: 4T.35.3218.16
(bootloader) version-cpld: None
(bootloader) version-microp: None
(bootloader) version-main:
(bootloader) version-misc: PVT SHIP S-ON
(bootloader) serialno: HT34PW913714
(bootloader) imei: 354436054211972
(bootloader) meid: 00000000000000
(bootloader) product: m7_ul
(bootloader) platform: HBOOT-8064
(bootloader) modelid: PN0710000
(bootloader) cidnum: ORANG001
(bootloader) battery-status: good
(bootloader) battery-voltage: 4315mV
(bootloader) partition-layout: Generic
(bootloader) security: on
(bootloader) build-mode: SHIP
(bootloader) boot-mode: FASTBOOT
(bootloader) commitno-bootloader: dirty-0e2a13e5
(bootloader) hbootpreupdate: 11
(bootloader) gencheckpt: 0