Give this a try: Go into your Date & Time settings un-check the time server setting and manually set the date and time so its correct. Now disconnect the power cord for the night (longer if you can). In then morning plug in the cord and check the date and time is is correct?
If not the PRAM battery is bad. So your settings are not being held so the polling proves takes place instead of what you defined. You’ll need to take the logic board out again as its on the back side [product|IF174-059]
[image|2525938]
As to why it drained, I’ve seen some issues with the M.2 adapters its also possible it was just bad.