About rolling over, baby will do when theyre ready to do it! I didnt even teach my baby suddenly one day he rolled his body over, but not able to lift his head yet cause neck not so strong. My 3.5m also having flat head, and now then i start taking action cs i panic, what i usually do is reposition his head left/right when sleeping altho most of the time he will turn his head back to the centre.
I think sleeping in a rocker/carrier for nap is fine as long as the baby is supervised, incase if baby fall from rocker. But other than that as long as baby can sleep, theyre comfortable can just let it be. (For our own sanity too, as long as baby sleeps).
Most importantly, diff baby diff pace dont worry too much!