Great training, jecht.
I like Columbus a lot. Really good race, great cause.
It's hard to tell how you'll do (i.e., how you might want to plan pacing) without knowing much more more about your other workouts. I saw the transition from speed to "MP" repeats, but I didn't understand the 7:20 (pace for 3:12, or are you going, as you said, for 3:25)? I hope you've had some good tempo/long tempo runs and other mid-week workouts with good blocks of mileage at a medium-fast pace. If you've also had some good hills, along with some core and strength work, even better. But I glanced at your link, and it looks like solid mileage and very smart work on recovery days.
I've done 3:12-3:16 with my 20- 22-milers averaging 8:05-8:25, but with solid long tempo and pace workouts, then I've also done 3:20-3:25 with 20-22-milers still feeling easy at 8:20 or so, but without having gotten as sharp in my faster workouts.
Overall, it sounds like you're in really good shape, and you have the right attitude about pacing at the start.
Even though Columbus is flat overall (compared to nearly all other marathons), you know that the rise at the start can sap you even without your being aware of it, as can pressing too hard the last mile around halfway or trying to hammer pace on the low rise around 18, 19. But you can just soldier on and plan to give up a handful of seconds each mile on those stretches. As long as you're smart about pace and have practiced a fueling plan, though, the last 10K is forgiving, with enough downhill to keep the wind in your sails without being steep enough to really trash your quads. It's a nice course for a negative split.
At any rate, in light of the concerns about pace and overtraining, I'd consider keeping the average pace for nearly all of your remaining 20-milers very easy, then using another one of your upcoming long runs (maybe a 16-miler?) as an opportunity for some pace miles, with a very easy 10-12 at the start and the last 4-6 around a goal marathon pace (as long as you're feeling good). You can get a feel for running a faster pace on tired (or slightly tired) legs without overdoing it. And that can be hard to do on your longest runs, especially if your body is not at all used to doing 20-milers.
Yes: Prioritize sleep, and respect the taper.
You wondered about 23-24. That's probably too much right now, given that you just did your first 20-miler, and your long runs are already a significant % of your overall weekly mileage. If you're doing 2-3 more of at least 20, including a 21 and a 22 would be probably be fine (the last, I'm guessing, on 9/29?), but I'd do them easy. Just get the great training benefits of a long, easy run. You'll be able to do the faster running on race day.
And yes: Pfitzinger should be a good read ... soon, if you can.
You're right about 2017 being warm, but the humidity was the really rough part that day (mostly near 90%). Most years, though, the weather is very good to ideal.
And you may know this, but if you don't: If you're in corral A, consider starting a few rows behind the 3:30 pace group. They'll go a little faster than you're planning to, but you can watch them stretch out gradually ahead of you as an easy way of keeping tabs on your pace. And don't worry if you feel really slow or lose some extra seconds getting bogged down in that first mile. You'll have a lot of traffic (especially in the narrow-ish start), but you don't want to waste energy trying to zig-zag too much.
Good luck!