1nm is the separation required laterally. Add the tracking tolerance of up to 9 degrees for no navaid plus the inbound tolerances of up to 30nm for no gear and that 1nm starts to get enormous.
For opposite direction passing, do you have a DME there or are you GPSRNAV/INS/AUSEP/ETC? What gear does the other acft have? You get zero nm with a sight and pass and to 20nm with some tricky gear. No gear, no sighting, 10 minutes after estimated time of passing. To get up before, 10 minutes prior to time of passing, nothing else.
Ground to 18000 will see anywhere from 5 to 20+ minutes before you are in need of a clearance. The other aircraft needs to descend from what ever level they are at.
When you do the numbers on just where an aircraft needs to be reference the dep/arrival aerodrome you will see that opposite direction has aircraft that are a very long way away from each other as issues.
In summary, how long is a piece of string.