I am also very sorry for your loss, but do think it is worth contacting your Insurance one more time, as it was an annual policy. Hopefully it will cover multipule events & if at the time it was taken out your partner wasn't ill, maybe they would re-imburse for the flights in question ?
I took out annual policies for some years due to travelling 2 or 3 times a year & assume that if I claimed against the policy for an incident on holiday one, I would still be able to claim for holiday two & three, if subsequent insurable incidents unfortunatly occured.
I don't think you will get anywhere with Thomas Cook as they are a business and is normal practice for travel companys not just TC. Others have reported similar sad occurances on HT & whilst unique to the individual a TUI/TC employee replied that they have such calls each week in their offices so will not make exceptions on compassionate grounds. No doubt not helped by the low-lifes who say someone has passed on just to try & get a refund because they have changed their minds.
I claimed from an Insurance policy in a similar instance & had to provide the death certificate and a Dr's letter (presumably in case I had picked up some random death certificate

I would give them a ring.
xx