You have asked the Professional Guidance Committee for advice regarding an ethical issue raised by your representation of the former client of a deceased attorney whose estate now demands return of the file regarding that client. The executors apparently believe the file is the property of the estate. The file's content frequently includes documents belonging to the client and the work product of the attorney representing the client.
Irrespective of ownership, you are presently in possession of a client's file which apparently contains documents regarding the representation of the client. As such the file is afforded the protection of a Rule 1.6(a) which states: A lawyer shall not reveal information relating to the representation of a client unless the client consents after consultation,.... Because you, as successor counsel, have responsibility for the entire representation of this client, that duty of confidentiality would extend to all documents relating to the representation and continues in all situations including a subpoena to produce that file.
This duty is, of course, concomitant with the duty imposed by Rule 1.15 to safe keep property. The comments to that rule recognize that a lawyer may hold property of a client against which third parties have an interest; apparently the case here. While you cannot unilaterally arbitrate that dispute you must, nonetheless ensure that the client does not interfere with the interest.
Certainly the file you describe may contain information which is not confidential nor owned by the client. No confidentiality vis-a-vis the client would attach to those documents and release to representatives of the deceased attorney's executors would be permissible under the Rules of Professional Conduct.