Stirling, IMO, minimizes/under-estimates the devastation caused by a wide-spread meteor shower (dozens? scores? of impacts, some quite large). A Yellowstone eruption will be quite destructive. No, sadly the Native American population is going to be decimated far worse in TTL then it was in OTL.
Indeed; given that there will be a layer of ash about a foot thick deposited across much of the Great Plains with lesser amounts covering the Great Lakes Basin. This is tantamount to an airliner encountering a flock of birds or suffering some catastrophic malfunction.
Understatement Alert:
It would have been a devestating blow for the young Canadian dominion as well. I'm sure however, that the U.S. wouldn't let something as silly as an international border stop us from providing relief assistance to our neighbors in need.