Assalamu alaykum Ustadh, When giving the tasleem at the end of the prayer, if someone mispronounces it while turning their head, can they repeat it once their head is fully turned to correct their mistake? Is that sufficient to correct the error?

Yes. Turning the head is a sunnah, not a condition, so the tasleem counts even without it, as Imam Ibn Baz said. If you mispronounced it by mistake and then said it correctly, that is enough and your prayer is complete. If you only doubt whether you mispronounced it, ignore that doubt.