Plug threads being wet shouldn't tell you which cylinder is having the issue since the plug seals itself into the cylinder. I've seen a lot of plug threads be wet..and usually it's a VC gasket. Understanding that yours are new and shouldn't be leaking, those threads seem odd to be wet.
As for fouling, you likely haven't run enough to show a good fouling from oil.
TMK leakdown test is only going to tell you if Chris did a good valve job. I don't see how it can tell you about the valve seal which would be next up from the valve sealing to the head.
The way I'd try to diagnose this problem is to pull both valve covers. I'd start by looking at all valve seals and all springs. Give it a good look over to see if anything seems out of place, to tall/short or has any rub marks. Thinking of how oil flows in our motors, can give you a lot of ideas on what to look at or test. Oil is under pressure at the cranks and cam bearings, gets pushed up by the lifters into the pushrods and then trickles back down over the rockers/seals and returns to the oil pan. It also passes through the rear engine cover, that typically shows up as a mix with coolant and shouldn't apply to being burned. Only other thing we didn't suggest is rings, while possible the rings on 3800's are low wear and great sealing. This is part of how we have these motors running so clean for upwards of 200K miles w/o rebuilding. Rings are one of those things that is a one off oddball. (Kinda like when I hung a valve, doesn't happen enough for us to consider it.)
