One tiny flaw in the theory:
I can see how one incorrect digit could lead to a 10 tonne error on a manual loadsheet...but you say the Crew saw this and departed anyway.
Surely if they spotted the error it was just a question of changing a 5 to a 6, or whatever, and, hey presto, no more error??
Or are you suggesting they knowingly did the calculations for the incorrect weight??
Sounds like BS to me.