So, after being finally so pissed off with Symbian^3 issues regarding to the use of QPixmap i decided that the fastest way to proceed is to switch to the raster engine.
Honestly, it works good on S60v5, where is the only one available, and i only do 2d graphics anyway. So, why not? I tried it and, magically, all the graphical issues, glitches and even crashes are fixed. I was able to go back using QPixmaps everywhere for the map chunks and blcks...

Speed seems to be as good, at least not worse, so i am doing some more testing then switching back to raster for good!