[color:"orange"]I loaded the game and showed FIRST to Trevor the note (i.e. Tingalf still remained unlocked by Robin)and also nothing happened![/color]

That shouldn't make any difference. I had Tingalf arrested before I entered Verdistis, and had no problems with Trevor. After accepting the quest to look into the suspected black market dealer, I just had to talk to Trevor again right away to be able to say I had Tingalf arrested.