So the guy in front is the one wanting it open, and the guy in rear is wanting it closed. And everyone agrees the guy in the front has the right to keep it open.

I think the etiquette should always be about who is being more inconvenienced. As far as airline shades go, anyone can always put something over their eyes to reduce brightness. But for someone who prefers brightness, they don't always have the ability to create more brightness (yes, I know most jets have little lights above the seats, but during daylight those probably aren't effective).

So I think the guy who prefers the shade open, should have priority.