Why would a shield wall be irrelevant against horse archers? Does a shield really care is their opponent is on their my little pony? Don't think so. Also, they were perfectly capable of making a round shield wall, so the mongols would just be wasting their arrows. That would mean they would have to go into a full frontal assault into hand to hand combat, were vikings excel. If the vikings were masters at anything, they were masters of one on one, hand to hand combat. You know this. The Mongols would just be cut down. Also, I could name plenty of commanders worthy of any Mongoloid army. Charlemagne, Richard the Lionheart (My hero), Henry V, Barborossa, Henry the Eighth... the list goes on and on. That was a pretty bull headed move of you to say that no European leader was equal to a mongol commander. Just because they rose the the rank by birth doesn't immediatly make them incompetant.
Also, how do you suggest they would get to scandinavia if they couldn't beat them on water? Think about it. They would have to go hundreds of miles into the frozen north, past Finland, than trudge their way across the peninsula into Scandinavia. It would be worse than Napoleans march into russia. Few would survive the arctic winter, even with their mongolian heritage. That would have been the deciding factor in any campaign.