I've watched many videos of professional dancers analyzing their dancing most of them think Jihyo is the second best dancer. I think you need to watch some of those ranking videos they are actually pretty objective.
Yes, Mina trained as a ballet dancer but that doesn't mean she is the second best dancer ,not to mention ballet dancing is so different from kpop/idol dancing it can a bit challenging for her because of the posture etc.
Yes but you never knows what goes behind the scene, surely Mina got upgraded to the Main dancer position for a reason