[Adger Smith]

Roller Chains don't work that way. The slack was probably caused by something wrong in the valve train making it go going harmonic.
Pick something else to blame.
I don't like revs in a free spin too high for the weight/strength ratio of the parts. That usually separates the valve train and makes it unstable, hence loose chain... There is probably other damage in the valve train you havent seen.
I've never seen a valve hitting a piston long enough to blue a rod/rod journal and loose a bearing. Valve head usually breaks off at the valve stem first.
ditto:
On lubrication or pulling the rod out of round. A long time ago.. close to the time you were born. Big block chevies had problems with rod bearings being too tight on the journal at the parting line and when the revs went high they pinched the crank and wiped all the lube off and caused the rod to fail/spin the bearing. I've seen a lot of that in the early days of my business.
I made some good money fixing them where they would last. It would be interesting to do an inspection of the pieces.

Another thing that makes rod bolts fail is excess load by negative G-forces on the piston-Rod assy. To put it bluntly the piston and rod trying to launch into orbit on the Ex stroke with no load. Resulting in a load that is too much for the strength of the bolt. Condition of the parts could determin if this was the case. Also if the burnt/blue of the rod extends into the bolts the heat could have weakend the bolt lowering it's strength and it's yeild point.
Just my .02