Actually mines are easy to explain if you look at it just a little differently.
They are not limited to the ecliptic either. Further a single mine unit is in fact a large group of mines not just one mine. Thus you have a largish wall of mines. And since most mines are put around one of the following...
A) A warp point
B) A planet
C) A star
Following explanation
mines suround the sector in a large globe, each mine unit being one globe of mines. Once one mine is activated the rest have simple computer seeking algorithms with one time use propulsion that allow them to target the offending ship and impact on it.
Simple easy explanation.
