2 is retarding the timing, S is retarding timing even further. If Brabus requires it to be at N it means they require you to use 95 RON. It doesn't make sense that retarding timing is necessary if you are running such a high octane as 99. I mean if you were running 90 RON I would say aha thats the problem. Very weird.

This is a guess, I don't know my knowledge is basic so here goes...what if the car has an over advanced timing causing knocking despite the high octane? So retarding the timing has helped it. Its possible that setting to factory doesn't work and you have to set it of slightly. Just guessing