Is an illegal screen a change of possession and a free throw if your in the one and one sitaution?

Yes, an illegal screen results in a change of possession. If the team committing the illegal screen is in the one-and-one situation, the opposing team will be awarded free throws, provided they are in the bonus. The player who is fouled will shoot the free throws, but if the free throw is missed, the team will not retain possession unless they get the rebound.