Thanks again.
Good news: If I enable the 16bit mode i can use the entire display.
Bad news: The issue about the doubled x mapping is still present.
I am bit concerned about this problem because as far as i know the "X-counter" is incremented automatically from the driver after it receives the right amount of data regarding one pixel; since the "init" seems correct i don't know what to check.
I will of course go through the display datasheet again trying to undesrtand if the "device_gr_fn" is sending data properly.