What should CO's staff have done different then - when the plane is full, the overheads filled??? Kick someone's properly sized hand luggage out (as they did once to me) ?

I just don't see how this is any different from any other cracked bag case? There is no space, oversized item must be checked. This is neither unusual nor a "PR disaster". Luggage breaks all the time, especially if you fly UACO. Guitar owner appears overly whiny that their noise generator got dented. That's all.