Thanks for the detailed analysis, I think this item seems likely to be the key:

4. Apparently the vessel had a swing keel, and if retracted, which it would have been at anchor, this would have reduced the vessel's stability yet further.

Captain to blame imo. Pure and simple. He should have been familiar with the design and thought it through to emergency procedures. Coupled with hubris of the rich/the owners (with dodgy past/s).