The first thing that I found that should be changed is the boundary conditions you have chosen PBC or periodic boundary conditions for every boundary which I don’t think is advisable in most cases since the power will never dissipate and the simulation domain will be filled with interfering waves. In your case the power would flow from the left to right and when it encounters the right boundary it will loop back to the left and enter the simulation domain again.
This could also be the reason why you are getting transmission power of greater than 1, since the power is propagating in a loop and passing the area detector more than once. I would change all the boundaries to absorbing boundary conditions to get an accurate simulation.