For me, a couple of key points:
explosion of mud prior to the blaze
blasting water onto a vessel in an “uncoordinated action” on any vessel, can cause it to sink.
They sank it. Then the riser broke apart.
I have thought a fair bit about this, and wonder if the FiFi would have such an effect on a semi, in which case the principal flotation is provided by closed tanks well below the WL. As the explosion and ensuing fire occured above the rig floor, I do not see how the pontoons could have been damaged.
Seems to me that all the enclosed topside spaces would have scuppers or drains, albeit undersized to cope with the water ingress from the FiFi. Perhaps the water went into the columns, as in photos that I have seen the DH took a steep list.
Do you know if she capsized, or went down keel first? Again, even if capsized, would not the pontoons, were their WT integrity not compromised, have kept her afloat?