And yet the furthest west part of the contrail (assuming a jet contrail coming from the horizon) is in shadow and in front of all the clouds in the picture. But you ignored the main point. Why is one edge illuminated and the other edge of the contrail dark? If the light source was from below and as far away as the sun wouldn’t it illuminate the entire contrail evenly?
There's no particular reason why it should be even at all. Just as the undersides of clouds aren't necessarily evenly lit. It depends on the perspective of the viewer and the position and shape of the cloud. The cloud (contrail) here isn't in a direct line of sight to the sun, but off to one side. Could be that the plane was descending as well, adding another angle to the mix. This would also explain why it just stops, too. The plane likely passed down to some altitude where the conditions were no longer right to produce a contrail.