(b) That's right. This type of case gives rise to substantial difficulties because both the negligent conduct by B and the effects of the later illness would be sufficient on their own to end A's acting career.
When a person is permanently injured but not killed, the court will assess the extent of the future harm that will be suffered. This requires making estimates about future events and factors, which is a difficult process. Courts do not like to simply speculate about future possibilities, but when, at the time of trial, actual events have occurred which undoubtedly affect the extent to which initial negligent conduct has impacted on the victim's life, those events must be taken into account. Then, once damages have been assessed and awarded, later events that occur are irrelevant.
Applying these principles to this case, B's liability will be less than it would have been if A had not also suffered from the effects of an illness that also made her unable to work.