2)Absence of vibration is a big clue. These things windmill at high speed, physical damage to the engine tends to lead to abnormal vibration levels.
I don’t know if they had vibrations
As a passenger on the plane, I can confirm that (apart from the times at which the surges were actually taking place) I could feel no unusual vibration. Once in normal flight, the plane seemed (to me) to handle exactly as other flights I have been on with all 4 engines operating.
But please bear in mind that, as I said in my first post (on page 27), I was sat over the right wing, not the left wing where the affected engine was. From my side I could neither feel nor see evidence of unusual wing or fuselage vibration, but I don't know for sure about the evidence from the other side of the plane.