OK, now I understand what you're getting at and I don't disagree actually. I also think that the original intent was that she died of a broken neck but the ambiguity is there, whether by design or accident, which makes other theories and later retconning possible. I personally suspect they made it a bit ambiguous to give themselves a bit of a back door in case the public would react too harshly to Spidey accidentally kiling his girlfriend. One has to remember how unexpected and grim this was at the time, it was a huge risk to take for the writers (Stan Lee even said later that he was tricked into OKing it while he was packing for a trip...not sure I believe that though).

It would have been easy to make her perhaps say something or make a sound when she's lying on the edge of the bridge, or make Peter feel her pulse to confirm she was alive before the fall. As the scene unfolds now, and the way she is drawn when lying on the edge (she looks dead), I feel its unlikely that wasn't intentional. But this is ultimately a matter of interpretation.