Eggshell skull theory

In what ways might defense counsel for a defendant (typically an insurance company's insured) who is clearly at fault for an accident attempt to diminish a claim involving subsequent injuries, including the exacerbation of a pre-existing condition under the "eggshell skull theory"?

Do written reports from treating physicians that validate a cause-and-effect relationship between the accident and the injuries carry greater weight than the findings of a defendant's Independent Medical Examination (IME)?

How does the "eggshell skull theory" influence the insurance industry's pre-trial (settlement) calculations for damages when treating physicians assert that the accident caused the injuries or exacerbated an existing condition?