I don't know if that would work as I read it has to be diagonal in the middle. But I put the plants there to hide the filter because I don't recall like the filters on view
I'm pretty sure you can put the heater either completely horizontal or vertical (unless the manufacturer specifically advises against it), and it doesn't have to be dead center. At least with my Eheims, that is the case. So long as you have some circulation going on in your tank, your temps should remain throughout your tank.