@Brian-McG Well, I just ran the route again, running the XT3 and MRA Next app on my phone in parallel. Success! I suspect (it's just my best guess) that skipping WP#7 manually instead waiting for "Skip Waypoints automatically" to kick in was the difference. The first time around, it really did take (in my mind) too long for the skip to trigger, which created problems around WP#8 that doomed the route. I agree that the "Offline Navigation" probably didn't affect what happened with WP#7, but I needed it any way for the second half of the route, where cell coverage was spotty and sometimes non-excitant returning back over the mountains.
As for the XT3, instead of just letting the route 'sleep' until auto rejoining further up the road, I instead selected off-route recalculate just to see the result. On the XT's I almost never issue a recalculate (that's another discussion), so it was an additional learning experience too. Like MRA, it recalculated back to the route as originally intended.
I'm sure we could have a spirited discussion on the pros-and-cons of a dedicated Garmin GPS vs. the MRA phone app. They both have relative advantages/disadvantages, and a lot depends on where and how you ride. For me, the XT3 will mostly be my first choice. But you're correct -- that's better left to a separate dedicated thread.
Anyway, thanks for your time on this. I've learned a lot about the app going through this process. Not all scenarios can be documented, and learning by doing is the only solution. At least I'm comfortable enough now to use either the XT3 or the app based on the situation.