Meikleour,
You write, "The biggest mystery to me remains as to why the crew thought that opening the crossfeed was of any use to them. In fact if they had done NO checklists at all, the outcome would have been much more benign."
My understanding of the report is that the crew originally believed that they had a fuel imbalance and by memory tried to balance fuel. This of course involved opening the crossfeed.
Some time later they determined that they had a fuel leak, not a fuel imbalance. Unfortunately they never went back and closed the crossfeed, thereby creating a situation that allowed all of the fuel to be pumped overboard. Other issues that arose concerned the location of the leak and the need to shutdown an engine (which did not happen).
Leaving the crossfeed closed or shutting down the leaking engine would have stopped the leak and made the landing a powered one.
The report is a good read and there is lots to learn for all of us.
best regards,
Bruce Waddington