[ramman434]

Finally an Update:

A friend of mine (employee of BMW) and I thought that the leak may be caused due to a faulty vapor seal, which was a plague of the E39s and some E60s. However, after a leak down test, the door appears to be perfectly fine, even though a bit more water than normal still appears to hold in the water exit channels.

Verdict is that we think someone had placed a bottle of water/gatorade/liquid, etc. in the rear pass. door pocket, without properly closing the lid. It had leaked over time into the pocket, creating the mold growth seen in the 1st photos, and then moved on down to the growth directly around the base of the pocket, on the inside of the panel.
The mold/water had sat there so long, that it had disintegrated the door panel, which was evident when the dealer pulled the panel off, as a couple of the cardboard bits that house the plastic screws (that adhere to the metal door itself) broke clean off.

Therefore, I'm getting a new door panel. I bought the car as-is w/no warranty from this boutique dealer and am trying to see if they'll cover the cost, since it was sold to me with a sh**load of white mold in the door. I'll keep ya'll in the loop.