-60mm covers will fit a range of earrings spanning roughly 57mm - 63mm.
-70mm covers will fit a range of earrings spanning roughly 65mm - 75mm.
NOTE: Covering earrings that are not sized specifically to the cover size may result in image distortion and could damage your earring, so please be careful and do not force covers that are too small around your hoops!
This is the answer to question 2.
This is the answer to question 3.