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Indeed, trying to rejoin the route from memory and turning too early is the only thing that really makes sense. If her GPS was working (and she had the route line on it, even with no mapping) she should have been able to get back to where she'd deviated from it.
Without the GPS working she would have seen the right turn (coming back North) in Gibbonsville and seen it was signposted Wisdom and may have thought that's the one.
From there it's a horrible game where doubt and uncertainty creeps in. This isn't the kind of road I expected, etc and the bad/panicked decisions begin.
I hope the latest tracker updates are her trying to find a suitable bivvy spot. Sleep (assuming she can) and then reassess in daylight.
Just guessing, but looks like Tanja has a Garmin Edge 1000 and no dynamo hub, so might have run out of power, though she has had a long break at Lolo hot springs lodge to charge everything less than a day ago.