I’m not sure that it is possible to avoid these kinds of discussion points. You specified, “as technical and scientifically logical as possible.” Malcolm’s reply was that: “The animal may be dead but its organs and cells may individually survive a bit longer.” This is pertinent to your question in that the same can apply to a leaf – death in the organismal sense occurs before death in the sense of cessation of cellular activity.
I had a similar question some years ago when we had a cat put down. I asked the vet, “How do we know when she’s gone?” The fact that the cat had gone completely limp did not tell me whether she was gone, because a cat under anesthesia would also go completely limp. What I was trying to ask – I now realize – was where is the point of no return? The difference between mostly dead and all dead (if you remember “The Princess Bride”)? Dead would seem to be – in its most basic, biologial sense – the point at which it is impossible to revive the organism. A cat under anesthesia, but not dead, can be revived; whereas a dead cat cannot, even though some cellular activity may still be ongoing.
If that is our working definition, then Anna Katrin explained it best: “the point of death for the leaf would be once those layers are completed and the leaf has been cut off entirely from the tree’s vascular system” – because that would be the point at which the leaf cannot be “revived.”