kayess, I really wish you the best in this matter.
I do feel MacCool has provided some good insight for everyone to consider. Based on what I've read (mind you, just what I read on my computer screen) it seems like you were under the impression that you had full coverage at the time of the incident, but perhaps that was not the case. If that plays out to be the actual circumstance surrounding your claim with the insurance company, you'll unfortunately suffer a substantial monetary loss as a result. A painful and unfortunate learning experience.
However, if fault lies with the insurance company and a mistake they made on their end (for example, dropping full coverage without your permission) you should be in a better position. The fact still remains that you (as the provider of insurance coverage for the vehicle) are responsible for ensuring full coverage is maintained until the title is released to you with no lien holder. If you received an updated declaration in the mail and failed to review it properly that is not an excuse the court will allow.
Since you have other vehicles on the same insurance policy...
Did you happen to call and make changes to the policy for one of the other vehicles? Could it have been a simple clerical error? Still, you're responsible for reviewing the changed policy.
In your most recent post you stated "Last Friday we met with the investigator". Who are
"we"? Did you have a lawyer with you? If you honestly feel you're in the right then you
need legal representation every step of the way on this. If found in your favor the insurance company will likely have to pay the legal fees you incur. However, if you lose the case the lawyer fees are sure to be on your dime.
Regardless, I wish you the best.