flight mechanics your discussion of steady state climb and dive is quite correct but you might want to mention unsteady climb and dive achieved by lift greater than weight a glider can make loops by pulling an angle of attack where lift is greater than weight as you point out this does not change the total potential energy except for the drag losses of course but you can climb without thrust my poorly trimmed hand launched gliders do it all the time at least until they reach zero speed and plumment to the ground