I've read about the diagnosis of PDD and DLB and it seems that the only difference between the two is when the onset of certain symptoms occurred. If PD symptoms occur first, then it's diagnosed as PDD. If cognitive symptoms occur first, then it's DLB. That was a bit of a surprise to me because I thought the PD symptoms started first. The neurologist didn't explain why he diagnosed DLB instead of PDD. At the time, I didn't know enough to ask the question. Ultimately, it probably doesn't matter.

Non motor symptoms
 

Hi Karl,
So sorry you are going through a bad patch.
I would agree with Robert that dementia is part of the PD package, not the other way around, particularly since sinemet "makes a huge difference". You say "This MDS neurologist said that the memory and cognitive problems I've been having aren't part of PD.", but there are no end of publications on the non motor symptoms of PD. I have seen 3 neurologists and they have all been vastly different in their views.
It is winter as John said, and we are all "under the weather"! I imagine Washington is a cold place at present too. Possibly a touch of SAD that a sunlamp could help?
I do hope you see light at the end of the tunnel, and can get out of the depression.
Best wishes
Ron