Not that this is going to help you, but insurable interest laws vary from state to state. Some states are like MD and require that the insurable interest exist at the time of the claim, but some only require that the insurable interest exist at the time the policy is purchased (and some are ok with either).
A little late for class here, Z, but I do agree with your clear analysis, Z.
However, I don't believe that the subject of insurable interests
has any relevancy here.
The OP has asked if her (ex, but extant) husband can single handedly cash out a life policy insuring his life. A policy, which we are told, was jointly purchased during their intermarriage and left unmentioned in the dissolution of the marriage.
So, acting on the belief that only the owner of a life insurance policy has that option. And assuming that the OP can produce proof of her equal ownership, (which, if untreated in the divorce would remain unaltered), and there being nothing to the contrary in the language of the policy . .
It is my opinion that it would require the participation of both the OP and her ex in order to surrender the policy and to share equally in its cash value.
(Adding, I am not about to visit a can of worms
as to any issues related to disproportionate ownership rights based upon the payment of premiums. And don't believe that the company - assuming that it will acknowledge co-ownership - would be so inclined either.)
I do believe that it behooves the OP as matter of preservation to promptly make known to the issuing company of her mentioned potential entitlements and that it best be formally communicated by her attorney. Also, the question of ownership may need to be resolved through litigation with her ex. But should he alone successfully apply, and the company is not placed on notice as mentioned, she'd be up the unnamed, but notorious creek.