Excellent point! My thoughts were that their presence were just part of the signs the Denethor would have to recognize that the Kingdom of the North would be renewed. It would definitely be up to him to acknowledge those signs. And his choice of arguing against the claim (which he did anyway, up to the moment of burning himself on the pyre) was certainly a choice Denethor would have to be ready to back up.
But I think Gandalf's point, subtle or not, would have been "go ahead and dispute it and refuse it, but be prepared for the consequences." I think there has been a former posting of one of the articles that points to that: "Mene mene tekel upharsin." Sauron had his signals. So did Denethor.
And I agree with you: Denethor would not have given up his role as leader so easily.
Note how Denethor sneers at his son Faramir being a "pupil of wizards" (or words to that effect). He knew that with Boromir gone, Faramir would inherit the Stewardship when Denethor died, and Faramir would be very unlikely to deny Aragorn's claim, or to go against the counsel of Gandalf.