iPhone 6 Logic Board Water Damage Clean/Repair
Perhaps the battery is dead and the U2 IC chip (controls battery charging) is also dead?
https://www.ifixit.com/Answers/View/177865/U2+IC+for+iPhone+5C+replacement

There is a way to bypass the U2 IC (though potentially very dangerous, but it sounds like you might know what you are doing).  

http://forum.gsmhosting.com/vbb/f631/iphone-5-charging-ic-jumper-done-1694941/

Ive also seen a youtube video where the battery was disconnected from the phone and charged directly... but i can't find it at the moment...