You can see the Canadian falls from the islands just fine.

Yes. From Goat Island, you get a clear view of the Horseshoe Falls across the river. Trees aren’t really the issue—the falls are massive, and the main viewing points (Terrapin Point, Luna Island) are deliberately cleared for sightlines.

If anything, the mist might block your view more than any tree ever could. Go early in the morning before the crowds show up, and you’ll see both sides without a problem.