ACMS you may know your figures for the 777 but you must admit you have NO IDEA what you’re talking about when it comes to the A380. Your initial statement that; "I'm told 12 hrs is at it's current limit with 450 pax and it can't carry any cargo with that number", is complete garbage. The first commercial route for Qantas is MEL-LAX-MEL. Sector time LAX-MEL is 15:25, so if we give 30mins to taxiing then you still have a 14:55 flight time. A hell of a lot more than 12 hours!!!!
So to run through your comparison on the 12 hour sector, with room for 3 hours more fuel there should actually be room for nearly 40 tonnes of freight if MTOW is the limiting weight.
Now I don't have an A380 performance manual in front of me and clearly neither do you, so we will have to believe that those who have spent billions of dollars on the machine know more about it's abilities than we do.