I might be able to help and maybe Jake can steer us both in the right direction.
We have the very similar symptoms, the same main board, and both our models are covered in the same service manual. My model is GDP615HSM0SS. In another thread, Jake said I needed WD21X26183 and I believe he is right. However, the GeApplianceParts.com website (and phone tech) insisted I needed WD21X25728 instead as it supersedes WD21X26183 which is no longer being manufactured. I took a gamble on the the WD21X25728 part as it was $20 cheaper. Just as you, I have no lights working with this part where my old UI did occasionally light up (and indicated the number 6 when entering CEM mode). And as you said, there is no Personality Jumper paperwork and I cant find any Jumpers on the board anyway. I also verified 13.5VDC and and also tried replacing the main board which didn't help.
I'm very curious if the successor part WD21X25728 is really compatible with its predecessor WD21X26183? Curiously, AppliancesPartsPros.com website suggests part WD21X26183 for both our models (and is available at $71) while GeApplianceParts.com specifies WD21X25728 for both our models at $43 as it doesn't sell WD21X26183 any more.
Jake, do you have any thoughts on this? I'm tempted to buy part WD21X26183 as you originally suggested to me (but not princekato), but wanted to see if I'm missing something here or missing some configuration steps with the newer part?