Secursus doesn't want that customs declaration out in the open, which is why they tell you to cover it up with the air-waybill. So taking it a photo of that will certainly not answer all questions about how the package was declared to customs, as you have said.
The bottom line is, the declared value for customs is of no interest to the insurer, unless they are using it to somehow create a loophole where they can refuse paying out on your claim. That appears to be what they were trying to do here based on the description of events given.