It could be from a number of different issues but two are very plausible one is persistent malware the other one is a corruption occuring on the flash chip by a improper software update or other type of damage that a security feature like verified boot or dm-verity would detect as unsigned, so ask yourself the following question very carefully: Have you installed any apps outside trusted sources like the Google Play Store? (1) — If yes - attempt to wipe the device from the recovery mode by navigating the available options displayed by holding down [Volume Down] + [Power] buttons while the phone is powered off. Hopefully the recovery partition is still intact, if this doesn't help try restoring the device with Motorola's official rescue tool https://youtu.be/TjsiGQkhVZ0 (2) — If no - it can be the result of damage to the eMMC in that case replacing the chip with a supported by the motherboard model/SKU **BY THE SAME OEM, WITH THE SAME PINOUT+DIMENSIONS ETC** and then attempting to flash the device...

With the text in the correct (readable) orientation flip the battery over from left to right or right to left, the adhesive strip will have (typically a pull tab) that when applying the strip on the bottom side of the battery should point downwards towards the repair technician similar to the application for iPhone 5, 5C, 5S battery adhesive

Likely a dead eMMC chip, if you've written a lot of data or flashed many ROMs it could have degraded to rapidly. The Nexus 5, Nexus 4 etc show practically the same symptoms when the flash chip is dead so one cam assume it's that.

it has to do with the proximity sensor, usually when you have flip to mute enabled in the my moto menu/app it shuts off the display in order to save power and mutes the incoming call. this can occur if the adhesive around the proximity sensor loosen and form a gap between it and the glass or ge debris or residue get on the inside or outside of the glass. the solution i have found, although a bit barbaric is to apply pressure AROUND the proximity sensor and whilst doing so taking the call. if this doesn’t help wiring a headset and taking the call with the remote is an option

all will fit in my experiance, the model numbers are a matter of where the device’s region should be. for example as far as i know xt1684 and xt1685 are native to europe ot reteu models. I have changed displays four times on mine and haven’t encountered model specific display assemblies or such issues.