I agree that I would try to work it out with the insurance company (IC) first. Other sources of help might be:
1) Your state insurance department,
2) The better business bureau,
3) I believe the American Council on Life Insurance (ACLI) has a policyholder assistance departement.

However, now that I've said that, I will also propose a scenario stating that this might not have been intentional by the IC:
You faxed your request letter on 5/16. If you did so before the market closed on 5/16, and the IC processed your request on that day (required to process within 24 hours by the SEC), your transaction date might have been shown as 5/15 to pick up the NAV as of the close of business on 5/15. In a "perfect world", both transactions would have closed on 5/15. In the real world, IC's might have hundreds of employees processing transactions. Just because your transactions were logged into the system in a particular order, does not mean that they were completed in that order. Frustrating as it might be, your transaction 13 might have been processed before the markets closed while 12 was processed afterwards.

I recommend that you:
1) Cool down and reread your prospectus regarding exactly what NAV is used when a surrender is requested - the end of the prior day or the end of the current day.
2) Contact the IC, they should make it good if they did act in contrast to the contract. If not, perhaps they can at least explain what happened.
3) Keep good records of your investigations. If you don't get satisfaction according to the terms of your contract, THEN start contacting the proper authorities.

GOOD LUCK!