This is pretty much spot on. I'd add that as the harmonic balancers start to get old and the rubber starts to fail they can tend to get a bit of a wobble, which is ultimately what ends up hammering the keyway. I think this can be made slightly worse by overtightened belts. The rivets in the pulley can also fail sometimes which doesn't help.I dont know why but I always like to guess...
Since we've seen this happen several times on different 2Fs, obviously the key in the crank is insufficient for the job when things get tough
Maybe decades of constantly changing velocity with a heavy harmonic balancer can slowly loosen up the key/groove to the point where it develops a tiny bit of play. Once it's loose, it's only matter of time before it slowly makes the gap wider and wider as it wears it away. Eventually the slop gets so large that the inertia of the balancer acts as a battering ram on the key which eventually shears the fatigued metal.
But I'm just guessing and am probably wrong. It seems that it would be prudent to replace the harmonic balancer at 150-200K miles to prevent this from ever happening.
Is the delta cam a rv grind? Or anything above stock? And how do you like it?