What I see there, and what I call it in the airplane world, is "bigger band-aid" syndrome. Guy finds a week spot (bowing transom board in your case), adds a huge ass reinforcement/doubler etc which transfers the load to another spot that was never designed to take a load causing catastophic failure. The area that is cracked is simply a splash well, it wasn't designed to have something structural depending on it. On my Lund I don't even think the splash well corners are welded, the corners are bent against each other and caulked before paint (either that or they're the smoothest welds I've ever seen). My Lund (and most of them), Princecrafts, others, have reinforcing gussets similar to what you've made Wendal, but there is another set underneath the splash well that they are attached to and the lower set is attached to the hull stringers. You had the right idea for taking the load off the transom board... you just need to pick up the underside of those brackets you made and get them tied into the hull stringers somehow.
Remember.. when the motor is pushing the boat.. load isn't an issue (unless you over trim nose up). It's sitting at the dock and trailering that cause the transom to pull rearward.