Shaka Zulu.
Just sit back and re-read what you have written.
Notwithstanding the whys and wherefors of the Captains actions, if a Captain, any Captain, is presented with a loadsheet he is legally obliged to certify that, to the best of his/her knowledge, the figures are correct. The buck stops there etc etc.
We all know that the ACTUAL aircraft load may vary widley to what is ACTUALLY loaded, eg estimated passenger/baggage weight, but the fact remains that one CANNOT sign for x number of passengers knowing that there are y passengers on board and then hope to escape censure when things go wrong.
Its obvious to all that the aircraft wont 'crash' with your 120kgs not accounted for, that is not the point and well you know it!
As for the jumpseat, be careful with your generalisations. Having jumpseaters and opening the flight deck door isnt the issue. The Ops manual will give guidelines for its usage and they are non negotiable; ours for example spells out the specfic occassions when it can be used and whose permission is required in such events.
Some rules can be interpretted and some most definatley cannot!
Thats why they are written down, to give you a reference to base your decisions upon - just as the Ops manual has step by step instructions for loadsheet completion!!
Its all a no brainer.
You may well wish to defend this particular chap but in doing so, dont make silly statements.