There are a few things here that would need more information/clarification. You say the airline AND trip insurance are requiring a police report. Who are you filing a claim with and for what? The airline should have primary responsibility here. They lost your luggage and it arrived sans items of value. When did you notice the missing items? Did you report that to the airline on delivery of the luggage and/or immediately thereafter? Are you filing with trip insurance because the airline is balking at reimbursement? Was the value of the lost items sufficient for all of your time and effort?
If a police report is required, it would seem you needed one from the local police where you discovered the missing items. You could have tried at the airport (most airports will have a police center), but of course you may not have been at the airport when you got your bags back. Getting a report would have been a huge hassle, especially since it was two days after the start of your trip.
While it is typical to advise never to pack valuable items in checked luggage, there can be cases where it is unavoidable. If you do have to check items of particular value, you should make a record of that and immediately inform the airline if it goes missing.
I would say to continue working and filing a claim with the airline. But if you didn't follow their procedures on lost luggage/missing items, you may not have much luck after the fact. I know that is a bit harsh considering you were on your trip and already had a 2 day delay of your luggage, but they also have to protect themselves against fraud.
I do hope whatever was lost wasn't of high value or "irreplaceable."