My friend has an addition on his house, and he had to have inspections done in a certain order. The plumbing leak test on the shower pan (Kerdi) had to be done before the walls went up. Therefore there was no way to fully Kerdi the shower before the shower pan had the Kerdi corners installed.
Now he wants to add the bench. I can't think of any reason that we can't Kerdi the wall behind first, and then just plop the bench in place on top of the finished Kerdi, without Kerdiing over the bench. There would be a lot of corners to finish on that bench, and it seems to me that since the bench will be installed on top of a finished Kerdi shower system, it's still perfectly waterproof.