Now I'm trying to cancel my original A-fare SK ticket from CDG-ARN with difficulty (will not let me do so online!)
Really? You should just have been able to re-book the flights from the original booking via the SAS site as A-class/SASPlus is flexible, as per my original suggestion. I've done this before, but I've never actually cancelled one. Anyway - you'll have your miles which is good
"Unfair skimming" implies that you are 100% certain that the longer distances are the correct ones. How can you be so sure?
Well, afaik they're calculated mathematically using the actual coordinates of the airports and trigonometry, so they should be as accurate as is it's possible to be - probably to the nearest mile. I doubt GCM or other websites actually have a lookup lists of all the distances. Anyway, it's a moot point as I think the ffp's have usually have disclaimers saying that the mileage awarded may not correspond exactly to the actual distances. If they're "skimming" it makes little sense to even bother in order to save 10 or 20 miles here and there.