[X Style (Pure) General] ResistanceIsFutile:
I'll try to help answer some if I can for you. I'm not experienced with mods and in general under the hood workings of Android. I do search and read a lot.

First you do know-

Craigslist is not a reliable source for most anything today.

Buy a used phone from Swappa or eBay with some kind of guarantee in what the ad states and you'll actually get. Then if a return period exists or guarantee period length in assurance you'll not have a lemon to live with in the end.

Both use PayPal for payments because it provides extensive buyer and seller protections. If the ad, device is not up to the advertising as was stated.

-On ebay look for an ad to state - eBay MONEY BACK GUARANTEE !

-Swappa's verification process includes running ESN checks and requiring the seller to post verification photos to prove device ownership.

Pre-Owned phones (especially one 3 years old) it's hard to buy used in knowing if you'll get one that has a Clean IMEI/ESN.

The Micro USB Charging Port might be bad in sending a voltage to the battery in someway (a guess) ? As the charger is feeding voltage directly to the device it works.

-The battery in the MXP is not an easily replaceable item as just removing the back off - there's a ton of glue, and connectors to safe guard in trying not to break.

You will likely want a new back cover assembly (door and adhesive)

You could try recouping some money by selling it on eBay - buy it now/auction for PARTS NOT WORKING in that type of listing. If at this point your done in working with this specific MXP phone or it's expense to you.

The file version in the OP states - Flashfile.xml shows: version="clark_retus-user 7.0 NPH25.200-22 21 release-keysM8992_1255331.29.01.88.09R", MD5 matches, and all dates are correct.

You miss that I'm guessing ?

Shortly after ADB/Fastboot flashing the file in OP, you should get a more current security update as I did. I don't know if these are in the correct order as I post them but, this is what I've seen after using "getvar ro.build.fingerprint". Yes as I understand, a security update will change the build number on your device the same as taking an OTA update will.

(bootloader) ro.build.fingerprint[0]: motorola/clark_retus/clark:7.0/NPH

(bootloader) ro.build.fingerprint[1]: S25.200-22-1/1:user/release-keys

--------

(bootloader) ro.build.fingerprint[0]: motorola/clark_retus/clark:7.0/NPH

(bootloader) ro.build.fingerprint[1]: 25.200-22/21:user/release-keys

As you can see in the screenshots I've included that lists security patch and build numbers.

About "flashing back to stock 6.0 is not a possibility since the stock 7.0.0 is already on the device, so there goes that idea."

I'm not versed at all but, have read that parts in flashing back are possible to jump back from Nougat to MM in using custom Roms like you want to of the LineageOS clark builds.

If I'm not mistaken- Just remember, if downgrading your Stock OS, always omit bootloader.img and gpt.bin from the fastboot queue of the firmware package files to be flashed.

Once bootloader.img and gpt.bin are updated, there is no reverting back without the serious and likely risk of a hard-bricked device. As to being confirmed by post #75

As it was - but, don't hold me to it... A stock firmware downgrade (in terms of recovery, kernel and OS) may be performed without any noticeable adverse affects.

This was prior to Nougat, so I'd advise you to check in whatever thread to a custom Rom your interested in to finding out any facts before hand. Notably kernel and modem related.

Acejavelin would know and his researching it enough to make that statement in post #63. His words are golden here as well as his experiences he shares to the XDA community. I appreciate his efforts.

Go ahead and search the web if your in doubt for Androids February 2016 update.

You'll probably see "A vulnerability existed that allowed someone who knew how to bypass the security measure that keeps someone from accessing your phone just by performing a factory reset."

Hope I answered you enough to help :good: :victory: