Hello Lutz,
Normally, I have little trouble in seeing your argument in this type of matter but on this one I strongly think you have it mixed up and wrong.
The wording of the licence regulation refers to scalar value of mass not the vector quantities of load or weight. It is not referring to the detailed mechanical capabilities of the actual vehicles onlythier combined MAMs. So if the maxim combined MAM (Masses) cannot exceed 3500Kg, then the take away the cars MAM and that leaves the maximum MAM or MTPLM that is available for the trailer subject to the 100% limit imposed by a another aspect of the regulations.
It's the difference between mass and force, I agree that if the car is loaded solo to its MAM, that means any additional MASS will exceed the MAM, and also the load on the axles, but without adding mass you can add load, (nose load) which does not increase the cars mass but does exceed the load margins.
Nowhere is load, weight or force referred to in this part of the driving licence regulations, thus load transfers have no bearing on the regulation, and after all it's a paper calculation, the other difficulty is if loads were taken into account; what value should be used for the nose load transfer? Its different for different makes of car, hitch and it is a variable for the caravan as it is dependant on load distribution.
The argument is largely academic as other real life factors come into play, but the point highlights the frequent misuse of units of mass force and weight.