Re: I don't know how often the backup verification run is performed but I suspect it is at least daily . . .
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I usually run the backup a couple of times a day and rarely trigger the "verifying backup" process. So, I would presume, it is a rather random thing depending upon the time of day or some such (i.e. #5 in your list).
P.S. I am running HIGH SIERRA since my early 2011 MBPro will not support Mojave.
