It also occurs to me that trainers can tend to agonise over whether we they did enough, whether we properly prepared our trainee. This has been mentioned already in this thread.
Having thought about this many times over the 20 or so years I have been training pilots, I have come to the conclusion that the mechanical process of handling an emergency is usually not the culprit - it tends to be more the subsequent decision-making, once the initial adrenalin rush has subsided a bit. "OMG I gotta get it down" turns into "well maybe if I do this or do that, I can get it back to base" and all the standard GA pressures start to operate (real and imagined). Having been back in GA for just a few weeks, I am already shocked at the pressures that exist to cut corners, and how readily my new colleagues submit to them.
In this case, I think that the pilot's initial handling of the emergency was probably exemplary, the radio calls demonstrate a level of confidence in the outcome, the details (like the lack of a mayday call) are largely irrelevant, but the eventual outcome doesn't seem to stack up when you consider the preceding events.
I guess my point is that safe flying is all about judgement, a skill that is learned, and unfortunately a skill that can also be subverted. No trainer can be held, or hold themselves, responsible for the decisions an ex-trainee makes.
Personally, I take the view that a light twin with a failed engine is essentially a glider. If you treat it like that, you minimise the requirement to exercise supreme skill in controlling a marginal aircraft in the worst possible circumstance. You might not make it quite as far, but your chances of meeting the ground under control are greatly enhanced.