According to my theory . . . your so-called "lift" from a sail is a result of changing the direction of the momentum of the air particles. This is why a flat board creates "lift." Those air particle that were once flowing toward your board, impact it, then leave at a different angle. Newton sez there must be a force that caused that -- and the equal and opposite force is what sends your board (or sail) off at an angle.
This is obviously the case with a keel, because the keel is symmetrical, so the path around one side is the same length as the path around the other side. The "lift" isn't because of Bernoulli, but because of Newton. The airfoil is more efficient because, as noted above (DDW?) it presents a path the air particles want to follow and so doesn't stall so easily.
D'Arcy's diagrams are the standard issue and are fine -- just don't leave Newton out of the equation.