Ok, upon checking my assembly instructions that came with my KK, I think I have found at least one of your problems... the bottom supports of the a-frame are sopposed to be bolted to the outside of the lip, not to the inside of the lip, that includes the two other smaller supports.
Bolting the supports on the inside is changing the angle of support for the a-frame.. instead of force being applied down, it has been deflected in. The area with the most stress is going to be the factory bend... as we can see on yours.. that is where it has started failing. That situation immediatly compromises the strength of the frame and leaves it open for the bow out like you see.
Basically this system is an arch... the bottom supports on the arch are too close. .. extra stress has been focused on the top right and top left sections.. they have failed and deflected out, this lets the top sink... in this setup, there is no longer support to keep the upright a frame arms from twisting out, as the unit is buckling. I would guess the shear weight of the cutter, pluss some quick fast and go mowing with a heavy tractor is what has caused most of the problems.
As for the issue with the top bolts on the a-frame and the 'U' piece. If it is missing, that just hastened the fatigue of the structure.. might have happened eventually anyway with the bottom supports too close.
The KK is deffinately not overconstructed.. they are designed to be just strong enough... defeating some of that precise structural design has obviously thrown it into the 'all bets are off' area.
If the dealer you bought this from assembled that unit as pictured.. you might have some recourse from the dealer or the manufacturer.. perhaps a new set of the two uprights, and the two small er supports, as they may be deformed as well. The 3-point lift pins are hardened and have probably survived.
Good luck. I know it isn't a good feeling to see a few undred bucks investment fail in a couple years..
Soundguy