The act should have two sections: the first creates a federal board to license journalists and media companies. Prior licensing of a constitutional right is allowed under DC v. Heller, 554 U.S. 570 (2008).
The second creates a 5-day waiting period before a licensed journalist or media company can report a murder with four or more victims. A brief delay before a constitutional right can be exercised has been held harmless many times. Threat of loss of license will guarantee compliance.
A mass murder will therefore be 'old' news when reported, eliminating the erupting mess of rumor and speculation we presently see with coverage-from-the-start. This calmer atmosphere will reduce the attractiveness of mass murder to infamy-seekers, who want to make the world spasm, saving many lives.