Firstly, we do not offer engineering consultation - we interpret Codes just like you do and implement our interpretations and those of our user base as changes (imporvements?) to CAESAR II and our other programs. So, simply because we show how the program can be used does not remove your obligation to exercise good engineering judgement. For after all, you are the engineer providing the evaluation of the piping system, not the program authors.
Yes, an older edition of the French code stated (I recollect) that changes in weight distribution is a secondary effect. This endorses your L5. So I agree with you but I am not the one you must convince. That French phrase has since been removed and I do not know why.
Standards come about from general agreement - when most people (engineers) perform a certain task in the same manner, a standard can emerge. Certainly, treatment of nonlinear response has not reached that point of general agreement.