Dear Stephen,
The bulkhead collapse theory is sensationalism peddled by the questionable Seanan Molony in his tacky documentary about this subject a couple or so years ago. Its amazing what some people will claim to make money. I think the documentary appeared on Channel 4.
His claims are laughable (that White Star was negligent in letting the Titanic go to sea and likewise the Board of Trade inspector, I think his name was George Carruthers) and offensive to the families today whose relatives died on the Titanic.
The compartment where the bunker smoulder, not a fire, took place was holed and so were several compartments to the stern of this compartment.
Therefore, compartments to the stern of the compartment where the bunker fire occurred were filling up with water anyway, so what does it matter of part of the bulkhead cracked. The ship was doomed anyway. If it did cleave, I doubt there would be a wholesale collapse of the bulkhead as it had strenthening pillars across it's whole length, so in reality, only a small part of the bulkhead would have cleaved, so I think.
Check the layout of Titanic's compartments and you will see this. I can't remember their numberings off the top of my head.
Maybe more learned people can correct me if I am wrong.
All the best,
Ajmal