Steffy, you can sometimes get a medical hardship/redshirt after you've had a redshirt, but you cannot choose to take a redshirt after you've used a medical hardship.
And to get the medical hardship you have to provide documentation after the season to the NCAA to get approved, so no you can't be clear to play and just sit out to get it. For Ertz and now Delton, it appears we know these are both serious enough to be season ending injuries and thus the medical hardship rumors.