If your husband was on duty in a police owned vehicle all of this should be being handled by whoever paid him workmen's comp. They will consult with him before they settle with the insurance company. I urge you to have your husband find out who to talk with. He can ask the Human Resources dept of his police agency. I'm a retired county officer and our worker's comp is administered thru the state.
If the other insurance picked up all the costs then he wouldn't have worker's comp. Still, since your settlement involves settling with the department's insurance, you need to talk with them about what they expect. They can also give you a better idea of things then we can.
Do not do anything until you get cleared to do so by his department and workmen's comp. And if you are allowed to settle then consult with an attorney. I've been in a couple of accidents, one of which sounds like it was as serious as your husbands. (in private car, off duty) The insurance company asked to settle and my insurance company suggested consulting with an attorney. With the attorney's help I received more than the company was at first willing to pay.
Since your husband is still having difficulty you do not want to settle now. Once you sign a settlement agreement there will be no more opportunity to get further money. You want to be sure that you'll have no more expenses. Insurance companies want to settle quickly because of this.
Do consult with an attorney. They can give you a better idea of what a reasonable settlement would be as well as when would be a reasonable time to settle. You don't have to hire them to represent you. It can be a consult only.
You said the claim settlement calculator sounds really high. You want to settle as high as you can. Your husband is apt to have difficulty with his back for the rest of his life. I had a whip lash injury 40 years ago and still have pain in my neck. I didn't settle for nearly enough.
Later: I do not know why his union would be involved at all. The union is there to provide work related support. An auto accident is not covered by a union contract. A union lawyer deals with contract issues. An accident is not a contract issue.