Last bit of info. . .
I realized that the two other events mentioned earlier where the hard-configured guards were unreachable were the result of local Internet connectivity outages --had slipped my mind when writing that message.
So the 30 minutes (almost exactly) of guard unreachabllity was a unique event following the hard-coding of the guard nodes. As mentioned, I've seen the guards change-out entirely before adding the EntryNodes configuration despite having "GuardLifetime 18 months" set.
Finally I checked the consensus votes for the three hours surrounding the event and both guards were in the consensus at all times and without interruption.