Unfortunately applying manned aircraft design criteria to UAV design would invariably lead to huge increases in design / manufacture cost. One of the attractions of UAV’s is the reduction in cost over a conventional manned aircraft.
The question of control v autonomy is a big one; simply put, it depends upon what the planned usage is of the UAV. Some systems require a pilot, with continued data updates. Other systems require a totally autonomous flight profile, in other words the UAV takes off its self, flies a predefined 3d (including altitude and position fix) route (normally through waypoint GPS navigation) and then returns for automated recovery. There are of course numerous permutations in between, sometimes the UAV mission will have the recovery and takeoff flown by a pilot who is in visual contact of the UAV at the initial and final stages, sometimes the fully automated UAV will receive mission updates which will reset the defined course and altitude. The number of permutations is as long as a piece of string.
With regards failure mode, that depends upon various factors, even with the fully autonomous control / mission systems, a failure will trigger a pre defined event, ranging from airborne destruction of the UAV (after a predefined length of time for loss of data link, be that for GPS navigation, sensor information or loss of power), to a parachute recovery, even to a deliberate end to the flight by a crash (normally caused by motor to cut off, and a deliberate spiral dive or spin) into the ground.
Collision avoidance systems are evolving, but at this stage the most common form of avoidance is given by not operating the UAV in an area likely to encounter manned traffic.
It is difficult to compare UAV design, both practically and operationally to manned aircraft.