Come to think of it. Maybe the "overspeed" might be a bit of a "red herring".
It's not unheard of for the bolted on counter weights to fail, I know of a very similar incident - similarly doing some pretty nasty damage. Have a look at the those two failed bolts closely on the counterweight you mentioned, see if you can determine the failure point.
The story I know of, goes like this. The engine, an MTU 2000, threw a counter weight not long after rebuild - which then proceeded to destroy itself. The engine had a complete overhaul as per specs. The contractor, a OEM authorized shop, failed to replace the counterweight bolts as required, which apparently have a very precise working life of 30,000 hrs.
Maybe your failure is the counterweight bolts, as suggested by the comment that you found them broken. In the case of dramatic failures like this, it's not uncommon for the alarm system to register / record multiple alarms, the first not necessarily being the cause of all your problems.
When was the last major overhaul, and what was done regarding the counterweights - whats required?
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