Or they'll use a pair of binoculars (or a drone with a camera) to ignore the decoys and shoot at the actual ship...

The horizon at sea level is about 3 miles. The strait of Hormuz is 35+ miles wide. Any mechanism used to get around this would be detectable and could be attacked with relatively inexpensive ordinance.

About 20 meter elevation would be needed to cover the navigable part of the strait. So, a couple of tall ladders?

Look at the map. They have mountains. Not much rain/fog in that area to block the sight lines. Maybe haze.