This is a great question. Thanks so much for asking it; I'm positive you're not the only one trying to figure this out!
Can I ask—you say you tried to take it but your name wasn't on the list. So you were scheduled for an in-person exam, you showed up at the testing center, and they told you they didn't have a record of your name or test for that day? Do you still have the email that they sent to you confirming that you'd booked the test?
I know that testing centers have had to cancel tests when stay-at-home orders have been extended in that city, but in the cases I'm aware of, my students did receive communication that their tests had to be postponed. I'm not sure whether that was via email or phone—but it's worth checking your spam filter to see whether you find anything there. Also, when you log into your mba.com account and look at the section that has your test appointments, what does it say?
Next, you don't mention the possibility of taking the exam online. I don't know whether you've tried practicing with the whiteboard already. I've spoken with students who tried it for a little while—a few days or a week—and then decided it wasn't going to work for them. I think it takes at least a couple of weeks of regular (daily) practice to really feel comfortable with it, but I do think many, if not most, people can get to the point where they feel fine with it if they put in the practice. We've had other students take it now, not just teachers, and some of them have even gotten official scores that were higher than any of their practice tests (including tests with the regular scratch paper).
I've been using it for about a month now. I just took the official EA Online a couple of days ago and I've gotten to the point where it feels natural. I didn't have to think about what each button / icon meant or which tool I wanted now or how I wanted to organize my work or anything. I was just able to forget about that part of things and do my work, same as picking up a pen.
It does take effort to learn, but you're practicing most days already / anyway, right? I would consider incorporating it into your daily studies now. If your June 6th exam is canceled, then you might find by mid to later June that you actually feel comfortable enough to go ahead and take it. Then you don't have to wait around until late July.
Also, I learned some news earlier this week that makes it a lot more of a no-brainer to just try it and see how it goes. GMAC is keeping Online scores separate from Test Center scores—you basically have two different scoring records and you report them separately to schools. So, if you take the exam online and don't like your score, you just don't report that record to schools. You wait until you can take it in a testing center and you report only your test center scores. (Or vice versa—if you like your online scores and not your test center ones, just report your online scores. Or report both if you like both!)
Okay, so now to your actual question.
Yes, ~6 hours a week is a good "maintenance" level—you'll be able to keep your skills more or less where they are. You will of course forget little details and more obscure rules, but about 3 weeks before whenever you are going to take it, you'll ramp back up again. You'll do a more detailed review / reminder of everything, take a couple of practice tests, and then you'll get in there and be good to go.
You have 2 more of our CATs? Have you taken any of the official practice tests yet? I usually recommend using one of the official ones as your last one, about a week before the real test.
You can repeat both ours and the official. It's possible that you'll see questions you've seen before, but if it's been a while, chances are you'll have forgotten most of them anyway so you may not even notice a repeated question.
In terms of content for maintenance mode, you really just want to be cycling around lots of different things. Then you'll see most things once or twice vs. seeing certain things all the time but then completely forgetting about other things. I'd do a random, timed problem set (use OG or our end-of-chapter problems in the books). For OG, you can pick out stuff to some extent just by glancing at the problems (an algebra, a geo, a wordy / wall of text problem, etc).
Then use the problem review to do a broader review—that's the real goal. eg, You realize that you were a little shaky on the exponent rule for this problem, so you go back and review
all the exponent rules in that chapter of the book. And maybe you set aside your exponent rules flash cards in a little pile of cards that you know you're going to review in the 2-3 weeks of your ramp-up before the real test. Basically, use the problem review as a jumping off point for stuff to go explore / remind yourself about right now as well as making a list of where you want to spend your review time in ramp-up mode.
Re: burn-out, ~4-6 hours is likely enough for maintenance—you don't need to do 8+. If one week, you're feeling it, feel free to go for 6 to 8. Another week, when you're not, give yourself more of a break. And if you find yourself really feeling burned out, go ahead and give yourself a few GMAT-free days in a row.
The other thing I'd try to use in maintenance mode is this idea of: I'm learning to play this game / sport. I want to be good enough to have a good game with my friends when we can get back together to play again. I don't need to kill it—it's a friendly game. I'm just trying to keep my skills up so that it's enjoyable when we get together to play.