in your opening post in the first and last pictures showing the underside of the piston - on the left hand side of the picture, the face below the gudgeon pin cylindrical housing area where the piston material has separated doesn't look as you would expect - i.e. more like signs of a flaw.
If we talk about a piston fail on that side why does the gundgeon backed up on the other side? I think that was the the first failure, the gundgeon pin was on the edge of the housing and the stress was to great for the material and made that triangular form on the piston.... Then the forces where all in the small end and on the left side of the piston and that's why the left side is in worse state the the right side.... If the piston cracked on one side the gudgeon would be still on the small end and the underside of the piston all smashed, or the small end should all smashed because of the rotation of the gundgeon pin... The small end is distorted because it smashed the cylinder....
not saying it was a flaw - just saying it looks different and should help you decide what happened - the more pictures for us the better chance of trying to agree with you..
In your fourth picture you can see what looks like the gudgeon pin has moved to and has been hitting the skirt where the recess is to actually get the gudgeon pin in . Its made its way out that far and has been wobbling up and down due to how far it has moved eventually pulling the skirt off . Lack of circlip . Thats what I can see anyway. View attachment 19116
