a) done everything according to the rules. In answering the question about whether Stephenson had given USS Evans sufficient warning about the imminent danger the Judge Advocate at his court martial asked "What was he supposed to do—turn his guns on them?"
b) they'd been there before.
Indeed this was the second escort that Melbourne had cut in two and sunk. Five years earlier in remarkably similar circumstances, Melbourne had collided with HMAS Voyager when the latter turned across her bow as a shortcut to assume her position as plane guard. 82 men lost their lives. Cue Lady Bracknell "To lose one parent, Mr. Worthing, may be regarded as a misfortune. To lose both looks like carelessness". Indeed so, but my reading of the evidence is that the carelessness did not lie at the door of either of the skippers of the carrier. Captain J.P. Stevenson, i/c during the second, USS Evans collision was so mindful of the potential hazard that he invited the Captains of all five escort destroyers to dinner at the start of the exercises, regaled them with the tale of the earlier disaster and handed out written instruction for how to avoid a similar incident re-occurring. As if that wasn't warning enough, on 31st May USS Everett F. Larson, another escort, managed a near-miss with Melbourne while taking plane guard station. Nevertheless, both Stephenson and Captain John Robertson (i/c Melbourne during the Voyager Incident) were given new jobs in shore-establishments and promptly resigned from the RAN, feeling dis-respected. In 2012, 43 years too late, the Australian government offered Captain Stephenson a written apology for his shabby treatment in an earlier era. On the USN side, the Commander Albert S. McLemore i/c USS Evans, even though he was not on the bridge at the time, felt that he was fundamentally responsible for the collision because he should not have left two inexperienced officers in charge of his command. My father would have approved.