Does a worshiper's head movement for the Tasleem, by mistake, during the last Tashahhud, and then correcting it and completing his Tashahhud before completing and uttering the Tasleem, affect the validity of the prayer?

The movement of the head during the taslim (salutation of peace) does not affect the validity of the prayer. Even if a worshipper performs the taslim forgetfully, he should complete his prayer, perform the prostration of forgetfulness (sujud al-sahw), and then give the taslim.