The report of the ARCCA experts is so damaging to the prosecution's case, I guess the only way to discount it is to assume bad faith and or outright deception. Which to me seems like quite the conspiracy in itself.You know that they didn't offer their report to the defense, right? The FBI offered their report. They of course agreed to testify, but that doesn't mean they are only in it for the money.
Yes, pedestrian/vehicle accidents do result in severe injury. However, he had ZERO bruises! Even the CW doesn't say the vehicle caused the head injury. So, if you declared that as your reason to find her guilty if you were on a jury, your bias should throw you off the jury. On a jury, you are to only discuss the evidence brought in to the court by the CW and the defense. The CW never asserted or proved that his head injury was caused by the vehicle.
I should also mention that it was never stated that the ARCCA experts were testifying "for free". What was stated was that they weren't being paid by the defense. It's likely that they have a fixed-rate consulting contract with the Feds (a very standard arrangement for professional service firms) and their testimony in front of the jury was included as a part of that contract.