I forgot if you mentioned this, but have you sought legal help? If so, you might be able to get the other person's insurance to pay for neuropsych testing. That's how I was able to get mine paid for. You might have to push for it, though. And the neuropsychologist here would not even schedule the testing until they had the money in hand. They said they had been stiffed too many times by insurance companies who pre-authorized the testing, but then never paid.

If you have hired someone to work for you, make them earn that money! Seriously, though, I'm not someone who goes to lawyers (the only other time was for my divorce), but I know I wouldn't have been able to deal with two insurance companies myself, while recovering from a head injury. Your lawyer can ask for a copy of the other driver's policy and see exactly what is covered -- that might help pay for missed work time for you, plus medications, doctor visits and tests, physical therapy, etc.