4 (a) That's probably correct. Currently, Australian law takes a fairly strict approach to causation. A plaintiff must be able to prove that the defendant's act or failure to act contributed materially to the harm. It is not enough for a plaintiff to prove that the defendant's conduct increased the risk or probability of harm, or that the harm actually occurred, because those things do not satisfy the requirement of a causal link between the conduct and the harm.
This rule can give rise to hard cases, and some judges have suggested a less demanding approach, but their views have not gained majority support.
In the present case, it is probable that the increased risk of sudden death from eating the toxic chemicals materially contributed to A's death, and therefore B would still be liable.