As a matter of interest what is the minimum thrust/maximum derate available.
I am surprised even a heavily loaded bus, even with max derate would have had trouble getting airborne on a nearly 4 km long downhill sloping runway.
(of course safety heights etc would not be met and it may waffle into the air - but it should still accelerate without dramas...)
As an aside - what are minimum unstick tests for? To prove that an early over rotation will still be able to get airborne ? Do they test this at MTOW?