2) Once I get my rating and endorsement, am I allowed to fly from the back seat?
Not familiar with UK regs, but a general requirement to fly from a different seat than the intended is that you can reach all relevant controls. In the back seat of the Super Decathlon there are neither prop control, mixture, fuel pump switch or fuel valve and therefore I would say "No, don't fly it from the back seat" (unless you have a qualified pilot in the front seat).
The problem is not that you can't fly it without these controls, but that you would never be able to carry out any emergency checklist.