Say, once the longboat was moored in Lake Winnipeg, the Norse fabricated smaller ships on site to explore the outlying areas. Those responsible for the Kensington stone were one such group.
Hypothetically, these smaller ships had a capacity of say ten to twelve, were shallow draft for navigating streams, but had a sail and mast for navigating the lake itself. Thus, ballast stones would be in order.
Such a craft could easily proceed south to the mouth of the Red, then work its way upriver for a considerable distance. Now, remember the topography between the Red and Runestone Hill -- extremely flat and poorly drained, as demonstrated by the number of artificial drainage ditches.
It is conceivable to me that, during the Spring flood, after snow melt, the entire landscape might be under water. It would be possible for a shallow draft boat to sail across a couple of counties to reach Runestone Hill.
Then, having beached at this location, it is also conceivable that the water began to recede. Once their boat wouldn't float, the crew was stuck -- unable to move. Because, now being surrounded by miles of marsh, they couldn't walk out, either!
This theory would seem to explain a number of otherwise inexplicable items:
1. The 14-day interval. If they were afloat the entire period, 14 days is a reasonable span to make the trip.
2. How the 220 lb stone got to the hill from wherever it came.
3. Why the reference to islands (hills above the flood) and why the group was stranded where they were and compelled to record their experience.