Ladies and gentlemen, I come to conclusion.
I made a small experiment of my own.
I cooked sucrose+water+citric acid+potassium sorbate+sodium benzoate thoroughly up to 95C and evaporated about half its water content(using seb saucier). this sample was called "evaporated sample"
then added the same weight of dry substances, and added water just to reach the final weight of the first sample. the substances were heated in a small covered cup in a microwave, so there was no evaporation. this sample was called "not evaporated sample".
I sent it again to the laboratory for a chemical test(Nestle LI 00-031-2).
The results were that both sodium benzoate and potassium sorbate lost about half of their quantity during evaporation.
I understand that this small experiment has no statistical significance, because I had only 2 sample, but that about ended the resources I could spent on the subject.
Preservatives should added at the final step of the cooking process.