You have given a extremely detailed explanation and you have done most of the possible stuff on software’s side but I immediately thoughts as soon as I saw your title that there is a simple solution: Replacing the dock connector. I am 99.9% sure that the problem rests there. [Link Removed] Please tell me if this works since I’m intrigued by the problem as well. Thanks
My answer is replacing the battery. If you watch Hugh Jeffreys, you will know that he had the same problem with one of the iPhone 7 he was refurbishing. Your problem here is that your battery is dead and unable to charge. [Link removed] After receiving the battery, find the right guide on ifixit.com and your iPhone will work!
Try doing a screen replacement. If that doesn’t work, try checking for bad connections on the Logic-Board. If you found some, try replacing the logic board. Personally, I think that the Logic Board replacement will be obligatory since the one your using will probably have board-level water damage.