in Yost today 3/19 (day 1 of conference play) it says,"Today is the last day to redhisrt players to give them another year of eligibility." I was always under the impression this needed to be done before the first game of the season? This would be crucial if it's possible. That's five games of some playing time albeit minimal PT. Is this legit?

A player cannot be given a redshirt if they have appeared in a game.

The Fifth game.............you have until then. You are correct in that they will grow more by waiting until RS them but they have to avoid appearing in any game.Except Exhibition games.