My concern with your plan is that, looking at how the stern rail itself is mounted, with the seat being essentially cantilevered inboard of the two sternrail mounting pads, you will definitely need some a support leg to take the weight.
So, if you have to add one leg, might as well add three. mount it directly to the coaming with 90 degree rail fittings:
installed to the coaming and the seat bottom, and stainless tube as your coaming mount legs, with a stainless tube leg tipped with a rubber foot as your third, inboard, leg. Add some padding to the rail as a backrest, and you're good to go. You can make your legs as tall as you want, and with the right height and seat shape access to the stern cleats is not compromised.