The bottom line is that this statement...
Bearing in mind that flying wings are notoriously difficult to keep in the air and need constant FBW input just to keep them from going out of control.
...Is technically wrong.
The flying wing design is problematic in the yaw axis, but that is solved decades ago. There is a false assumption that 'constant FBW input' means human inputs. The reality is that the fly-by-wire flight controls system removes the human pilot from that problem, at least with US designs anyway. What this means is that if human pilot inputs are absent, the flying wing UAV will revert to programmed responses, whatever they are. But it does not mean the flying wing UAV will enter uncontrolled flight, crash, and there is a wreckage of aircraft parts on the ground.
At no point did I say that FBW requires human input. It can also have autonomous input, like the going from point A to point B, assume certain flight path at B etc. as you said.
But a flying wing usually needs input from FBW to keep it in stable flight.
What I was saying was that I don't see what would have to happen for the RQ-170 to malfunction in such a way that it lands on its belly in almost perfect condition.
Communications? Uh, no. American engineers are not stupid. In such an eventuality the RQ-170 would have used protocols to return back to Kandahar under autonomous control.
Flight software (FBW)? Well. The aircraft would very likey be uncontrollable and wouldn't land in such good condition.
Engine? The pilot would know he couldn't make it and crash the RQ-170 into the ground in order to prevent it falling into enemy hands intact. Bearing in mind the RQ-170 can fly quite high, there is enough energy to point the nose down.
The only thing I can think of is that both the communications and the engine would have to fail (maybe the communications first or simultaneously, since the pilot could have time to recognise the RQ-170 won't make it and act upon this knowledge). And Lockheed Skunk Works would have had to be dumb enough not to incorporate a self destruct mechanism or procedure ("descend to sea level nose first at maximum thrust").
IMHO the stars would have to align in a more unlikely way than Iran jamming the RQ-170's communications - setting the UAV on a return path to its home airfield using GPS - and then conducting a GPS spoofing attack, thereby sending the UAV to where Iran wants, while the UAV thinks it is going to Kandahar.