Simon you have my greatest sympathy on your engine struggles. I can perhaps answer one of your questions, the damage your pictures showed can definitely be related to a knock. Just last week I replaced 2 pistons in a street driven 4 cylinder engine for a low knock at idle and on acceleration with just that kind of damage. I also found nothing else wrong with any bearings, rods or wrist pins. Two new pistons ( in this case stock size ) and a light honing of the cylinders cured the problem. I and my service manager believed this to be a lack of
oil problem on this particular engine. good luck Frank