A player becomes the holder when a legal scrimmage kick for which he positioned the ball on the ground IS MADE. When no legal scrimmage kick is made, there is no Scrimmage Kick Play and therefore there can't be a holder as defined in Rule 2-27-3-b ("During a scrimmage kick play ...").
When a "holder" is tackled while in possession of the ball, he is (only) afforded the same protection that any other runner has (Facemask, Spearing, and so on).