Just a couple of foot notes to your good analysis:

The family was visiting Las Vegas and lived in Pennsylvania, a vastly different and more settled environment. I am speculating, but I'll wager they had no concept of how sparsely populated the North Rim country can be. While I'm at it, I will further speculate that they conceived of Arizona as a desert, which we all know to be hot, and did not realize that the only thing between the North Rim and the North Pole is a barbed wire fence....

Again I assume they flew to LV; hence they were driving a rental car which wouldn't have had any of the random bits and pieces that can be found in the typical family vehicle, often useful in odd situations. I am confident that my vehicle, parked outside, is better equipped for winter survival than their rental...

I'll bet the concept that a part of a hugely visited NP(6 million visitors in 2016) could be closed was utterly foreign. And, everyone has heard the stories about how hot the Grand Canyon is - you never hear about NR snowdrifts.

The notion that climatic conditions in the intermontane west have more to do with elevation than with latitude is foreign to many folks who have lived elsewhere.

My last trip to the NR was to participate is a "marathon." We were going from the NR to SR via the kaibab and bright Angel trails. We spent the night at the NR in early May, just after the facilities had opened for the season. Beginning our run, we dodged snowdrifts still on the trail. As we descended to phantom Ranch, the temps flirted with 100F..

The Canyon is wild and gorgeous, but it is rugged and wildly variable. If you venture there, you need to be ETS.