I’m thousands of miles away, and a decade or so out of the came, but I still know the game, and I doubt there are many “castings” that are fracturre critical below the deck. Yes, the steel drill casing below grade loses ductility in the temps and pressures we’re talikinag about, but not like you seem to me to be describing.
There are reports that pressures might be from 20 to 70 thousand PSI. If true, nothing we’ve got can stop it.
You are right - my engineer friend’s suspicion is that the casing string parted at one of the cross-over assemblies as the casing string tapers down from a larger diameter to a more narrow diameter at depth. If they had a bad cement job, there could be a mechanical shift/jolt sufficient to part or shear the casing off of elements in the cross-over assembly, or disconnect a key element in it. Or maybe the whole thing parted and including the surface casing string and got shoved upward - if the surface string casing hangers got shoved up into the BOPs it would explain why they failed to close the blind rams - no way to penetrate such heavy steel even with the rams. I imagine there has been some casing collapse too downhole given the loss of control issues around the completion and well blowout when they displaced mud to seawater.
Best chance to control is to intersect the reservoir with relief wells and frac the reservoir massively with cement.