The story about the aircraft taking off using only half the runway is a furphy, initiated by faulty tracking that only commenced once the aircraft was halfway down the runway. The runway is 3,500 metres long, and a Dreamliner needs 2538M to take off.
Add in high local temperature at takeoff, a full aircraft at near MTOW, and it would need all of that 2538M to get airborne. No-one in their right mind would take off at the halfway mark of the runway, with a full commercial flight at MTOW on a stinking hot day.
If the aircraft initiated the takeoff at the halfway mark along the runway, it wouldn't have even been anywhere near V1 when the runway ended.
The CCTV shows an aircraft climbing satisfactorily for about the first 500 feet, and the questions to be answered is why the undercarriage wasn't retracted as soon as it left the ground, and why the flaps appear to not be set to takeoff position.
The flaps may have been extended for takeoff, the video quality is too poor to determine what their position is. It seems impossible that an experienced Captain would takeoff with no flaps extended - that is so basic to aircraft operation, and there are so many warnings if you try to, the chances of that happening is near zero.
MenTour pilot has a fairly good discussion about the flight on FB, get past the first few minutes, and they get into the nitty-gritty. It's all speculation anyway, until the FDR and CVR are examined. Both have been recovered.
https://economictimes.indiatimes.com/industry/transportation/airlines-/-aviation/air-india-crash-flight-ai-171s-last-takeoff-analysed/articleshow/121811830.cms?from=mdr
https://www.facebook.com/watch/?v=1207383097733798