Can Veil of Summer force an opponent to target their own creatures with Swift End when Lucky Clover is on the battlefield?
Lucky Clover's trigger is not optional, your opponent has to copy Swift End. But they don't have to choose new targets, even if the original target has become invalid by the time they copy it.


  115.7d If an effect allows a player to “choose new targets” for a spell or ability, the player may leave any number of the targets unchanged, even if those targets would be illegal. If the player chooses to change some or all of the targets, the new targets must be legal and must not cause any unchanged targets to become illegal.


If they don't choose new targets, the copy will not resolve (because its only target is illegal) and they won't lose 2 life from it.