Cargo was much heavier than what was shown on manifest
Can't happen.
The Loadsheet is done from weights off a Pallet Statement, ie every container is weighed.
Manifest is just a nice simple crosscheck for any SNAFUs, and will give you a net weight of freight, add the containers on and you should be very close to your total Pallet Statement weight.
As for shifting units, I cannot see it, having dealt with the loaders at MEL, they are professional and even an oversight such as leaving a pallet lock down wouldn't cause a weight shift enough to cause this. Worse case scenario would be a shift in an all but empty Cpt 3 and 4 all the way aft. Very unlikely.