that is among the better and detailed diagnosis's i've seen in a while!
My guess is the actuating rod for the clutch is at fault? this doesn't explain the explanation for 1st gear though, However, first gear is pretty low and may not slip if the clutch plates are just hardly touching.
either way, I'm figuring it's in the bell housing, there will be better than I come on here with ideas soon probably.
I wonder if there is something slowing fluid return to the resevour or perhaps some air pocket that heats up lessening the clutch pressure?