No way that I know of to get DL miles from UA. Used to be able to do some fancy footwork when Hilton had Reward Exchange with DL. Not a great ratio but at least it offered something. Amtrack exchange only with CO? miles and now severely limited to 25k/year? Even the outrageous points.com doesn't allow such transfers.
From France to Douala--I travel that route fairly frequently IN THE BACK. Econo fares are cheap and it is not that bad for 6 hours with reasonably comfortable seating and seat pitch. On all but one occasion, I have always had an empty seat next to me.
Although it is tolerable in the back, I certainly enjoy getting back on upgraded flights on Air Canada across the pond. ^
Good luck getting points to Delta AND securing a reward.