I was a passenger on this flight,
I'm not a pilot but I have made moer than 100 flights in several types of aircraft as a passenger.
I was in the rear of the cabin at seat 34C and I watched the takeoff from the seat monitor via the forward camera. The aircraft seemed to rotate a very late and run out of runway. We did strike the end of the runway and there were visible sparks from the window near me. I did not see any engine issues from my side of the cabin.
The rate of climb once free flight was achieved seemed very low. There were some very distressed passengers and the crew didn't seem to be much better! I still have the fingernail marks in my arm from the lady sat next to me!
The captain admitted a technical fault after being in the air for 10 minutes and the flight over water to dump fuel seemed to take forever. I must admit I was more worried about landing as I thought the landing gear might have been damaged.
The landing happened without incident and the flight deck should be commended for that.
I must say it felt to me like there wasn't enough thrust to get the aircraft into the air. It took too long to get up to speed and then by the time we had got up to speed there was no runway left! The tail scrape as you call it did happened three times to my memory.
I must say I'm glad things happened they way they did and no injuries occurred. I am very interested in the investigation report. I don't think it was pilot error and more likely misinformation given or a technical fault with the aircraft. I wish the pilot could have slammed the brakes on before the point of no return but thats wishful thinking.