Final update, just got back from the shop yesterday after a final conductor plate and plug socket swap. While I had the valve body out I sprayed electric contact cleaner to clean out the plug sensor thing and it looked good. By the time I had everything back together and back in the transmission though I noticed more oil on the plug, all while the plug wasn't even touching the transmission. I was already planning to pull the TCM anyway to clean it out, so good thing I did as the entire case was filled with transmission fluid. After I originally changed out the conductor plate and plug housing it must have sealed properly. I believe ATF just wicked back down the wire from the TCM since I hadn't cleaned the TCM out on the first go. That would explain why the outside of the plug housing wasn't leaking like the original oem one had, it was only leaking inside the actual socket. All is well that ends well though. The new plug socket fits a lot more snug and feels much more secure, and the genuine Mercedes conductor plate did look a lot more high quality than the non-genuine one that was in there before. The car is running perfectly! All that is left is fine tuning the transmission fluid levels, it was difficult to get the transmission over 80 degrees in the shop, so I will probably wait to test it cold and go off the cold reading on the dipstick, I have 5L sitting in there for now.

Moral of the story: Make sure to check your TCM whenever you do a conductor plate swap!!! That would have saved me a couple months of heart ache. A friend of mine actually has a 1999 e320 as well and we put his up on the lift just to check (no limp mode problems or anything) and his plug housing was leaking very badly as well. We pulled the TCM, and it too was leaking fluid. It wasn't as bad as mine, my board was completely submerged in fluid, but the bottom 1/4th of his TCM was coated in ATF. So it might be worth checking just as a preventative measure.

Thanks again for all the help, and thanks RPM55 for pointing me towards the TCM!