[Verizon Galaxy Note 3 General] bftb0:
First, bear in mind that even in the case of stock retail firmware bundles, Samsung has never distributed "complete" firmware sets (including things like PIT files, efs/pad/param/fsg/persdata/persist partitions, etc). So if you want take that risk filled journey of a full flash wipe, make sure you have a backup copy of everything in the first 22 partitions from your phone PLUS the slack space between the beginning of mmcblk0 and the beginning of mmcblk0p1 : in there is the protective MBR, the UEFI primary GPT, the PIT partition data, and even some undocumented MD5 data. It might be a good idea to also capture the slack space behind the last partition as well (at a minimum the UEFI secondary GPT is back there).

Note also the one of those partitions very likely contains handset-specific factory tuning/calibration corrections for the radio as well. Throwing them away in the hopes that something generic will be better seems like a bad idea.

I don't know of anyone using Odin who has successfully overcome the anti-rollback protection of the bootloader. If you consider what a "developer device" is intended for, it is highly doubtful that "bootloader development" is one of those activities (except for Samsung engineers, who would be using engineering firmware, not retail builds).

That is, the "unlocking" is for things that would normally be flashed in the Odin AP slot such as boot.img, recovery.img, system.img, etc; not bootloader-layer firmware.

If you want to test this out, I would advise you to first try very low risk stuff - a normal Odin flash of MI9 bootloader firmware only (no ROM software), with a twrp*-4.3 recovery installed. (Rooted-Stock N*- and O*- kernels will not boot from MJ7, so that's doubtful from MI9). The idea would be to preserve some form of bootable root that would allow you to re unlock the MI9 "aboot" from TWRP. (Also, CM13 is bootable from MJ7, so I suppose you could leave that in place as your dirty ROM and you'll have two chances at root preservation)

One of two things would happen: you would find out right away "no way Jose, Anti-rollback is still in effect with the unlocked bootloader", or the flash would succeed and you'd have a pre-rooted ROM in place to effect the bootloader unlocking from (no need to re-root). Worst case in the second alternative would be that you couldn't boot the ROM or the recovery, and you'd have to flash the MI9 stock AP slot stuff and follow that up with what.... RDLV? (Man I wonder if the flashables are still available)

There never was such a thing. Otherwise anyone with a retail phone of the same release (or not subject to AR protection enforcement) could just flash in Odin to convert their retail device to developer mode.

What makes this exploit possible is the fact that the CID was regarded as a hardware specific key, so that simply copying firmware from a DevEd phone to a different retail phone wouldn't work, so long as the CIDs were different. And then it got uncovered that the CID could be changed, so now we are all walking around with CID clones (but not IMSI/MEID clones) of some fellow's DevEd phone.

You know what? I'm one of the few people with a MJ7 phone. IIRC, it can be rolled back to MI9 even with a locked bootloader (AR protection doesn't block that rollback).

You might want me to test bootability of some recoveries from MI9 before you get started. And I in turn might want you to test a few things, e.g. this. That might be in your interest anyway - if the MI9 b'loader is finicky about kernels that it can boot it might be prudent for you to try booting a rooted MI9 ROM+kernel from later firmware (NC4/NK1/OB6/OF1) before you start attempting bootloader firmware rollbacks. That might allow you to have a compatible but pre-rooted ROM in place if the rollback actually "takes".

Send me a PM. I have other approaches (that are a bit more dangerous) in mind as well.

PS. After saying all this, I have to tell you I am skeptical that your reception issues are related to ROM software or even Modem flashing. I don't doubt you are faithfully recounting your experience, but what are the chances that the phone just happened to get dropped or damaged somehow right around the time you flashed MJ7? (Post Hoc fallacies seem to run rampant around this place)