Napoleon would have won easily in the North because Koutouzow had already pointed(correctly OTL) that Napoleon's aim would be Moscow;
If he marched North,he would have the entire Polish cavalry protecting his right flank and they could harass the Russians in their approach...
Polish cavalry was very good, but I doubt it would be decisive by its lonesome. Russia had very effective light cavalry forces itself, and matched up pretty well against the Poles.
Secondly, the Northern Force was already there to shadow and block the French from attacking new directions, something they did very well. It gave several battles, most of which were very bloody for the French, achieving the strategic goals of the Russian command. Northern Force couldn't stand up to the Grande Armee put together, but it could buy enough time for the Russians under Kutuzov to catch up and give them a Borodino on the way to St.Petersburg.
Finally, yes, any coastal route is going to be harassed by the Russian navy, and non-coastal routes through the Baltics have incredibly poor logistics at the time.
Going for Moscow was a huge mistake, but it's not like the Russians didn't have contingency plans. Going for Petersburg might not be any better.