I would imagine you could do it similarly to how a motion blur effect works, just with a much slower update-rate.
No.
A shader only has access to the current textures. Motion blur does not have an update-rate, thus you cannot slow it down.
Clones is the way to go.
Unless you want the trail to have the CURRENT frame of the sprite (that is, the sprite and trail change frames at the same time) in which case I remember writing a variation of motion blur that had a trail, but that's also limited to a boundary around the sprite...... short story, it's best to use clones in that case too.