G,
Every time I see a post from you it is high calibre!
That answers the question nicely about the implications on the structure and it would not be unusual after take off to be 30% over Max Landing Weight....so my sim instructors concern is clearly a valid one.
My other half of the question concerns the risk associated with staying in the air on one engine dumping 50 tonnes of fuel. I heard recently (cant remember the source) that certification requirements of key components are 10 to the power of 9 (assuming this is accurate and relates to engine certification) then I guess the minimal chance of a second engine failure (in the extra half hour you were airbourne) could be offset against the seriously high chance of structural airframe damage.....what do you think?