Toasted my engine

"Yes, but that wouldn't have sheared the bolt at..."

I agree not sheared but it would have looked like that to the OP. I
would guess a tensile fatigue (probably initiated by bending) with the
crack starting on the side closest to the cyl centerline for cyl 3.
Assuming it is truly one of the bottom studs - they would not be
thru-studs but short ones instead. My guess is a partial loss of
preload of the #3 cyl assy initiated by the failure of the thru stud
between 2 & 3, then causing a progressive failure at the bottom of
three. There probably are also some cracks around the base of #2 also

I can't explain the crack at 4.

If the cyls are reused the flanges around the bottom should be subject
to very careful magnaflux inspection. Maybe on general principle they
should be junked.

Torque to loosen will be less than torque to tighten, and less
indicative. The engine history would be interesting. It certainly
would have failed in a few minutes rather than hours - and it would
have been a massive noisy failure too.

A Bonanza friend found one of those short studs laying in the cowl
while preflighting his airplane in the Bahamas. He put the family on
commercial airlines & flew home on minimum power - with a life jacket
on!