Three more notes...
First, please let me say a hearty "Thank You!" to those who posted such nice responses to my trip report. It was a bucket-list vacation, for sure.
Second, if you know someone who is disabled, encourage them to travel to Glacier National Park and drive the Going-to-the-Sun Road. Even if one is confined to a wheelchair, the GTTSR is a stunning visual experience.
Third, the Steri-Pen worked for my son and me in challenging circumstances. While in the Scapegoat Wilderness (part of the Bob Marshall Wilderness Complex), my son and I used a Steri-Pen to treat our drinking water. We had tablets as a backup but the Steri-Pen did its job.
The entire region is blessed with abundant wildlife.
We saw significant amounts of scat of animals both wild and domesticated---grizzly bear, black bear, elk, dog, horse, and so on. (The area is heavily favored by horsepackers.)
We actually drew drinking water from a fast-running, clear stream whose banks were liberally festooned with cow manure---some fresh, some not so fresh. We were careful to draw from the middle of the stream, where the water flowed most swiftly. (The trail to get into the Scapegoat Wilderness goes through 1.5 miles of cattle-ranch land, then through another one or two miles of National Forest lands that are open to grazing. Also, there is horse traffic everywhere along the trail. Therefore, I consider all surface water suspect.)
The incubation period for Giardia is (from what I have read) 5 to 15 days. I have been back for almost a month and have experienced no adverse effects from my Montana sojourn.
Therefore, I believe that the Steri-Pen is a worthy addition to any backpacker's kit.
Again, many thanks for the positive comments.