trainer actually puts higher premium on visibility from the back seat than a two seat combat aircraft because it requires the instructor in the backseat to be constantly on top of what the pilot trainee in the front seat is doing in the air, And see what the trainee sees. A two seat combat aircraft would only require the WSO to fly the aircraft on rare occasions, and for visual look out giving the pilot better all round visible is more important than giving the WSO a better vantage point to aid in visual search.
the fact the no major airframe modification seems to have been done to improve rear seat visibility, as had been done on two seaters of SU-27 series, would actually IMHO lend weight to the theory the second seat is primarily for the addition of a WSO, and with dual trainer demands being the less important consideration.