The Chicago Open is in four days. Assuming a player continues his daily regimen of reading chess books for a few hours and playing two, perhaps three slow games (60/5, at least) a week, what else should a player be doing?
you should be playing more and studying less. Your study-to-play ratios are fragged. Acceptable ratios (suggested by "everyone") is somewhere around 4:1, maybe even as high as 50/50 if your opening play is solid. I know you advocated a 20:1 ratio in previous posts, and that is way too high. You need to play more, see more positions under fire.
Also, should a player stop this regimen before the tournament to "clear the mind"? If so, how many days before?
What ever brings peace of mind and reaches your comfort level.
I'm completely confident in the middle game. Everyone says that opening study should be reserved for when I'm higher rated (~2000).
This is true, because by following general strategy, I can transition into the middle game easily. With a plan, it's hard (if not impossible) to go wrong.
Why, then, does everyone advise to review openings before a tournament? (I have taken perhaps a brief look at the classical variation of the Ruy Lopez, but that's it.)
"You have to choose the systems you feel comfortable with and understand their principles." - Chess instructor
Define "Everyone". you should know enough to play an opening comfortably. I suspect you are somewhere in the  1499 range (edit: based on the Chicago Open Entry List and your name, you are rated 1499) based on your previous questions. Studying openings before an event won't help all that much. Mental quietude and rest is much more valuable in my opinion. My name is not Everyone.
Good luck at the Open. We look forward to seeing some of your games here on Stack Exchange.