What is the ruling on one who moved his head to say the salutation, then remembered that he had not recited half of the last tashahhud, so he brought his head back, completed the tashahhud, and then said the salutation?

What the person did, by catching up to the rest of the Tashahhud and performing it before the Tasleem, is correct, and his prayer is valid. The head movement that he made, which was out of place, is considered a minor action that does not affect the validity of the prayer.