How should one sit on a bidet? Either way, whichever is most comfortable. You should sit on the bidet with your feet on the floor either side, either facing the spray or with it behind you. The directional spray of water can be directed at where you want to wash.
Is it possible to fit a bidet back-to-wall? Yes. However, specific plumbing requirements will need to be met in line with the measurements shown in the Twyford Specification Manual.