How does Alpha Strike targeting and damage splitting work?

You target one friendly unit and any number of enemy units at battlefields when the spell is finalized. On resolution you split damage among remaining legal targets and must assign at least 1 to each target you originally selected. Alpha Strike can hit units at all battlefields, so a moved unit remains valid. You choose targets when the spell goes on the chain and cannot add new targets later, so a newly Ambushed Teemo is safe.