Sharp conclusion. Clearly conrod bolts have not backed out slowly.Yes, rod bolts backing out would do it but if you look the intact rod bolt looks seated, and damaged on one side only indicating no spinning, but it still hit the C/B'er enough to machine a trench in it. Whatever happened it got worse as the final hit looks, as I said before, on the rt side of the C/B'er cheek - it's the one that bent the con rod cap hole ( on the side of the sheared off head rod bolt ) - you'd think all that crashing of parts together would have made a hell of a racket but that's as deep in the engine as you can get so maybe not -
This is also interesting theory. Everything can happen indeed.I have seen the big end break and engine still runs. The crank would break but would not come apart. Because its mounting would not allow it to seperate.As far as the bolts backing out. High unlikely both rod bolts did that. But anything's possible.
What comes to BC shaft itself it looks like it have weak spot. There goes large M10 bolt thru and serrated portion is only approx.15mm diameter outside. So wall thickness is only few millimeters there. Balancer mass portion is quite heavy and if that M10 bolt is loose serrations have to bear all the torq what comes to rotate its inertia. No axial force hence no friction to contribute resist torq. In the case of open bolt shaft fatigue strength is limited. So maybe that serrated end has started to twist and leading mass part out sync.