It looks like excessive heat to me, too. So now the question is... Why the excessive heat?

And the answer is likely just a worn out engine. As the bearings get worn in the crank, the cam and even the wrist-pin things can get out of tolerance and all the adjustment in the world will not help it. But if these tolerances get too far out there is more friction and that means more heat and that leads to damage.

The rod bending would lead to a heat build up and that could effect the other pistons that were close.

It looks like it is time to call in a professional machine shop for the real answers.