[Android Q&A, Help & Troubleshooting] TheDrive:
Update:

Earlier in summer I have fully discovered IMEI/MAC/etc ID storage locations and found the ways to recover them with a binary patch.

This unique phone (unless most other similar Qualcomm-based devices) has a very good feature. It has encrypted EFS/NVRAM partitions (ModemST1/ModemST2) seems to have binary structure identical or close to the reference Qualcomm design. But OEM have made extension to the modem FW. There is one more important partition in the eMMC called nvbackup. This partition contains most important NVRAM setting required to reinitialize NVRAM in the case of the fatal failure.

Binary data structure in the nvbackup is proprietary to the Casio-Nec OEM, however this partition is not encrypted in any manner. For a people familiar with a binary patching there is no problem to find and patch any ID's (IMEI/MAC/Serial/etc) to recover originals lost due to e.g. foreign full eMMC image uploading.

When the main EFS/NVRAM storage is intact, phone will work OK with ID's found there. Once EFS becomes some kind damaged modem FW will erase it and immediately reinitialize the new NVRAM inside. Then NVRAM will be filled with the default values, and ID's will be copied from the nvbackup partition. That's why many users reported (in android-forums etc) they have had ModemSTx partitions erased and got a working modem no other problems, that is simply impossible in any other similar devices. Once encrypted ModemSTx (EFS/NVRAM) some kind damaged it will bring any other device to the inoperable mode with lost IMEI and non-working modem (SP connectivity will be completely impossible). That's why this phone is unique and NVRAM failure-proof.

So to restore your original IMEI/MAC/etc just:

1. Get the working nvbackup image (from other's full or dump from your phone),

2. Find in image and patch all the ID'd you want to restore with the values from the device sticker.

3. Flash [back] nvbackup image to its place (partition). in the device.

4. Erase ModemST1/ModemST2 partitions (make the backup first to be on the safe side!)

5. Reboot device and check modem is working and ID's are in their places.

The big and detailed articles on the theme how to find and patch the particular ID's and other details you can find in the topic on the 4pda.ru Russian forums mentioned above. (in Russian, use google translator.) Look closer to the last pages (May-July 2016)

Moreover!

Yiou can use standard Qualcomm QPST application to work with NVRAM of this device through the Qualcomm Diag COM port.

Use QPST -> Software Download -> Backup/Restore interface to backup or restore the whole [main] NVRAM image to the Qualcomm QCN format file. You can binary patch this file before restoration to fix your ID's. There are many instructions around the internet how to patch QCN with the correct IMEI. You can also view QCN file and export its structure to the human-readable ASCII text form using QCNView utility (provided with QPST installation).

However if you will fix the main NVRAM with a correctly patched QCN uploading you will get the ID's (IMEI) you have written in. and your device will work with them unless EFS/NVRAM failure will occure. On the failure NVRAM will be reinitialized as I've described above and your ID'd will be filled in with the values from the nvbackup. Probability of the EFS failure is very small unless you do not perform 'massive' (dangerous) modem FW related 'experiments'. But this possibility persists. anyway. That's why it's preferred to patch nvbackup image neither QCN to permanently recover ID's.

Now upon a disclosure of all the most important boot and modem related facts, data structures and required code, this device can be recovered, probably, from ANY damaged state and condition, related to the SOFTWARE failure. All you have to do is 'heavily' read and try to understand thousands strings of the multiple articles. There is enough information to recover this device but many procedures are complicated even for an advanced users, who familiar with many advanced techniques and tools. 'The man can do anything' say people in my country. I've done the exploration and disclosure of all these complicated device internals even though I've never seen this device nor touched it with my hands. Sure, you will can fix this device once you willy want to.