Hi! Good questions. In order:
(1) Anything you put into the online whiteboard will stay throughout the official exam, even if you open and close the whiteboard repeatedly,
unless (a) you erase it yourself or (b) the proctor has to reboot the exam for some reason. I would set up what you want at the beginning, but be aware that if the system freezes or for whatever reason has to be rebooted, then you'll have to enter in your time management strategy etc again.
Also note: On the practice whiteboard available on our website or on mba.com, the above is not true. If you close the practice whiteboards, they will wipe clean.
(2 & 3) When I took the online exam, I was able to open it during the instruction period, yes. (Note: They have since switched to a new proctoring company, so it's possible that this could have changed.) I would go ahead and open it whenever it first shows up on the navigator bar at the top of the screen. (Note: If the proctor tells you that it is too early to start using it, then stop using it and wait until whenever they say you're allowed to start using it. But my assumption is that we're allowed to start using it whenever it's made available to us in the software!)
(4) I also took the GMAT Online on my mac and I did not have issues with my enter key—it allowed me to go to the next line when typing. I would suggest reaching out to customer service now to ask about any issues you experienced on the official test—before you take it again. It may be the case that they can put you in touch with someone to troubleshoot before test day.
(5) I've just checked mba.com and that site does not list any keyboard shortcuts for navigating the official GMAT Online. You can use keyboard shortcuts when taking the official practice exams at mba.com, but I don't know whether they are available at all or universally available during the official exam and, if they do exist on the official test, I also don't know whether they are the same as what's in the practice exams. Given that the GMAT Online needs to work with a number of different operating systems from users all over the world, it's possible that this is inconsistent and so they don't provide a list because they don't want to promise that any shortcuts will work for everyone. In short: I wouldn't count on being able to use keyboard shortcuts to navigate the system.
You do have to use the mouse to select the answer bubble that you want (ie, your hand is already off of the keyboard), so might as well just continue the motion to click next and confirm.
If you haven't read this yet, check out our blog post on how to use both the physical and online whiteboards for the GMAT Online:
https://www.manhattanprep.com/gmat/blog/online-whiteboard/That post has our best recommendations on what to do and when to do it.
Also: The most time-consuming section to set up for time management is Quant. If you're choosing the section order Verbal - Quant, then I would only set up the Quant time management stuff if you are able to do so before the Verbal section starts. Otherwise, I would wait until just before the Quant section, just in case the test does need to be rebooted and you have to rewrite everything. (Or, if you finish Verbal a couple of minutes early, set it up then before you go on break.)
Good luck on your test!