The thought of mounting the bench over the tile is a bad one. Regardless of who says it's aloud or O.K.
Work is work. It's hard. Trying to make a hard job easier will net you a weaker shower. You will be stressing the wall tile with the bench when you and your wife use it. I have seen many of these benches go in online and I would suggest that the bench go in now before waterproofing. I would then tile the bench first and bring your cuts to it.
I would leave 1/8" room for expansion and silicone this gap. This will be a tank. A little more work but a much better build.
If you decide to attach the bench to the finished wall tile I would pretreat the sides where they meet the tile. To pretreat the sides I would run a thin bead of silicone 1/16" so that the metal does not make direct contact with the wall tile. This will act as a compression gasket. Also with this approach you will need to ensure there is no voids behind the wall tile. It could crack.
Between you and me I would never go this route. Just because the manufactures says it's OK does not mean that it is O.K.
They say you can install Kerdi and tile right away as well. Anyone who has ever pulled a tile up while setting knows the suction and force needed to do so. Anyone tiling over fresh Kerdi can not check the coverage or risk pulling the Kerdi right off the floor and wall. Industry guidelines say to check coverage. It's a no brainer. So because Schluter says it's OK does not mean its right. Same principle here with the bench.