My understanding has always been that unless the pilot's operating manual (which forms part of the C of A) states that the aircraft can only be flown solo from a certain seat, rear/front or left/right then as long as he/she can reach all the controls then the solo pilot can occupy either seat.
In relation to dual flight or instruction, the pilot in command should be able to reach both controls, but oddly for example, there may be some instructors may not be able to reach all controls, for example in a PA28, the fuel cock from the right hand seat, so should in that case they instruct from the right hand seat. This is a muddy pool.
It's all down to good practice and common sense, operate within the POH and you should be covered in terms of insurance and any subsequent litigation.