The gemoro in a number of places (like tamid 29a) say that chazal exaggerated about things. The rambam in הקדמתו לפרק חלק talks about the important role guzma plays in divrei chazal.
yayin stam is mederbunun so sufek derabunun lekoola is the general rule but i would agree that as a mumar his yayin is passul even if he was a jew.