[In the UK]
We would usually re-order the line and say, "If it hadn't been / had it not been for her (and her quick thinking) they would all have died." ['her and her quick thinking' is a bit quaint.]
But, preserving your order, we might say either, "They would all have died if it hadn't been / had it not been for her and her quick thinking", or "They would all have died but for her and her quick thinking."
In "They would all have died except for her and her quick thinking" the words "except for her" create a
false scent: a misleading expectation created by ambiguous writing.
We are mislead into thinking they all perished except her. It is only in retrospect, having reached the end of the sentence, that we understand that they all survived.