Banning eating pig and blood (both important sources of calories), banning alcohol (light beer and cider was pretty much the only safe thing to drink), fasting which followed a weird moon calendar, which fit very badly with their lifestyle, a legal system which to large extent ran counter to their own and no political benefit in converting. yes I guess the Europeans could convert to a version of Islam, which remove anything Islamic, but I can't see a reason why.
Well, the Hanafi school (followed by 1/3 of Sunni Muslims) only considers wine and other grape-based alcohols haram. Alcohol derived by means of honey, barley, wheat and millet such as whisky, beer and vodka are permitted in the Hanafi school. So the ban on intoxicants wouldn't really affect the Kievan Rus or Vikings.
Alcohol was never the main source of water in ancient or medieval times. Natural bodies of water, and well water, were the main sources of water, which is why waterborne illness was such a big problem.
For the most part, historically, people didn't even realize that water could be polluted until the 1800s unless the water itself actually smelled and tasted bad. Even when London's sewer system was being redesigned in the 1860s they thought bad air and miasma rather than bacteria and toxins were what spread disease, although they did correctly identify contaminated water as the root of the problem.
Islam with its' dietary laws in general and Ramadan in special is ill suited for more northerly latitudes. It simply woudn't do if a considerable portion the more religiously observant part of your population dies from dehydration whenever Ramadan happens to fall within 30 days of the summer solistice. So this pretty much rules out Scandinavia. Other regions that would have been unlikely to convert to Islam are those where viticulture was economically important so I'm rather doubtful that Italy, France or Greece would have converted voluntarily.
Dehydration? In Scandinavia, which has hundreds of thousands of lakes and rivers?