You don't indicate what state you are from, but you are clearly aware there is a state statute that applies. Whatever it says, don't go beyond its strict terms. Formal estate administration is intended to protect people, both heirs and creditors. If there is an exception to a requirement for a formal probate process, it is always narrowly construed.
If the checks are payable to the estate, you have neither the obligation, nor the right to convert them to a cashiers check payable to someone else, regardless of what the attorney says.
Never take advice from the other guy's lawyer.
Whenever you believe legal advice is necessary, ask your own attorney.
My suggestion would be that you deliver the checks jointly to one of the heirs and the attorney who claims to represent them. Get a receipt.
If they negotiate it, the endorsement would be the responsibility of the institution that accepts it for deposit. Hopefully they will require the appropriate appointment of a personal representative.
However, if the attorney is so sure of his legal position, he can just go back to the insurance company and have the check reissued, right?
In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.