Can you post photos of what the prolapsed tissue looks like?
Keep the tissue cleaned and moist with an ointment, oil or honey, you don't want the tissue to dry out.
Get some Calcium Citrate with D3 at Walmart or CVS and give her 1 tablet daily. Calcium is essential in helping with retention.
Keeping her in a dark area is not going to stop her from laying eggs unless it's total darkness for 16 hours each day.
Not sure why you want to feed only corn and water, is she in prison or being punished?

Give her her normal feed, she can have some egg or a little somethin' as a treat.
Tissue is not going to stay in unless it's starting to stop swelling. Daily soaking in a warm epsom salts bath may help with this as well, but you only soak if the hen is alert. Dry her well, then reapply your ointment or oil.
Edit to add, I do see where you had 2 other threads with minimal help. If you can get photos of what the tissue looks like today that would be good.
https://www.backyardchickens.com/th...ith-my-chickens-behibd.1568895/#post-26644822
https://www.backyardchickens.com/threads/something-is-wrong-with-my-chickens-behibd.1568895/