If the Emperor places a wall on an arrow, can Sinbad still place a cube on that arrow?
And can said cube move "through" the wall to the other region when the plage piece comes marching in?
Not sure on this one. The only clarification is on the Courier not being able to reach them.
First point is that Sinbad can't move his pieces onto the arrow if the Wall is there. The coast is shut down and he cant get out. That much I am certain of.
The question for me is what happens when pieces are out that and then a wall gets placed on that arrow. My guess is they are still allowed to move because they have to be on one side of the wall or another. They can't just be trapped there. But there is no mechanism to free them unless you take the Emperor and move the wall off the arrow.
So my guess is that once on the arrow, nothing stops them from moving off the arrow. But if a wall is placed on an arrow, it is not a legal arrow for Sinbad for adding new cubes.