Is it assumed that the predominant terrain (via art) in each hex dictates the terrain for that hex?
I assume so, but some hexes (like N9) seem to be drawn in a way that might imply otherwise. In the aforementioned hex, does a unit traveling along the road undergo the MP penalty from Long Range Artillery?
I hate ambiguous terrain and this is the worst kind of ambiguous. Majority of the hex N9 is absolutely clear terrain. But as you point out it looks like the road is in the green part of the hex so you could argue it's in the woods. But then again the green portion where the road is doesn't have the woodsy splotches so is that an area that has been cut down to allow for the path? Who knows? Just recommend coming to agreement before the game.
In less ambiguous cases, yes we play majority portion of the hex rules.