I'm not sure about everywhere but in the U.S. there is an app called "Airmap" Link to webpage. It's built for commercial drone operators but it can also be used by people flying for fun.
There is a flight plan portion of the app that allows you to file for a flight. You specify time of takeoff, radius you'll fly in, altitude and aircraft. This is then ...
ADS-B Out units are getting pretty small for reporting position.
Example from uAvionix.com: https://uavionix.com/products/ping1090/
Requesting takeoff clearance could be done via an automated system, where the operator enters takeoff time, time enroute, planned route, etc. Enter it some minimum number hours ahead of time for review and approval.
The downwash is directly related to total lift; more rotors just means each rotor is producing a smaller fraction of the total, and the end result should be no different for separation purposes than for one or two rotors.
There are a lot of obstacles to mass adoption of personal aircraft, but the exact lift mechanism is arguably the smallest.