The ERAU accident involved a competent and professional FAA DPER, John Azma, who I knew well,, from Orlando Executive. The failure he had was not foreseeable by the crew and was not survivable, shades of a certain Macchi loss of Willie many years ago. I had a sister ship tot eh one that was lost, but was far fewer hours and cycles, and we grounded my one from testing subject to detailed inspection. Losing a wing is a permanent mess to your day.
As far as who calls what, when the exhaust system fell off Glenn Todd and Kell Aldridge's aircraft and took out my propeller and silenced Pete Nalder for most of the rest of the flight, trimming his nose somewhat, I seem to recall that I put out the mayday for both aircraft, and funny enough, no one actually complained about that at the time or subsequently. Years later, sitting around Dave Stevensons hangar at Corona ("Dave crashes better than anyone I know...") 3 of us in the hangar had lost props in flight, and not one of us was complaining about the mayday calls that we had made.
The Mayday call gets a bit of peace on the line, and gets some attention to what the guys are going to come up with next, is is supposed to.
As far as controllability checks go, they were -1 and NATOPS standard procedures, seeing is nice but the plane will talk to the driver and suggest a course of action if they have the sense to listen and apply simple "CDF" to the matter at hand.
Regarding a wing falling off messing up your day, that's exactly right. But why mess up someone else's day as well? If you're in doubt, then you should act on it. So hugging the coast and asking for a runway to be inspected after landing indicates there was some doubt. Diverting to Avalon would be a way of avoiding the potential to mess up someone else's day.
Regarding the Mayday call, I agree it's quite appropriate for the lead to call Mayday on behalf of both aircraft. But in this case the focus seemed to be almost entirely on the downed aircraft. I could be wrong, but Melbourne Control advised Moorabbin of a possible emergency. No mention of Avalon as an option when there was a possibility of a damaged aircraft flying over built up areas for either Moorabbin or Essendon. I'm just saying perhaps it would have been a good idea to include Avalon as an alternative. Not sure why so many on the forum seem to have issues with keeping all options open.