Yes it's a nice theory.
The Vikings, as experienced seafarers would have had many other indicators as to their azimuth than just the position of the sun.
The direction of the wind and swell would have been obvious to them also. How do you tell the direction of the wind without the sun to orientate you? There would be several indicators, temperature of said wind not being least of those factors as well as the cycle of the weather patterns, smells of the direction from whence the wind comes etc., etc., etc.
I was in the south pacific ( I'm always in the south pacific but I mean the tropical south pacific islands) last July and commented to my 15 year old son that the wind had changed. He said " yep, Southerly's in".
A Canadian chap staying at the same remote island location just looked at us quizzically, the spot we were in was sheltered from the wind and it was not obvious at all that the wind had shifted or which direction it was coming from. He had no idea of his bearings but my son did, like I said we live in the south pacific and just 'know' what a southerley feels like.
Further to that,The island chap who took us to secret reefs to dive on knew instinctively where they were, no GPS, cloudy days, very few landmarks (none that we could see) but heset a course and stuck to it and dropped us right on the reef. Uncanny.
Newfoundlanders find there way home from the fishing grounds in the thick fog by following their noses, literally.
Aboriginal Australians can find water in the desert through sophisticated 'songlines'.
Ernest Shackleton found a remote Whaling station in South Georgia in a busted arsed life boat with no humanly reasonable opportunity or conditions to use his sextant from
A shitty little spot called Elephant rock near the Antarctic in conditions that would have made the Vikings' hair turn red (!). Do read about this voyage, it really is quite amazing.
Astounding!
Anyway, nice post Sandrine.