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Answers.
How does any of these help a pilot understand how to skillfully handle a heavy transport at FL350, let alone provide a meaningful level of control proficiency? How many pilots ever manually maneuver a heavy at FL350 under normal conditions when all indications are normal, correct and expected?
It gives them the confidence to handle an upset and a good idea of how to recover.
Considering the surprise, suddeness, and confusion surrounding an upset at altitude, coupled initially with disbelief and the likelihood of missteps that compound the problem - what are the chances that a typical, "experienced and trained" line pilot will recover safely?
Much improved as the pilot will be better prepared and less likely to suffer any 'startle effect'. Also, see answer above.
Would the amount and content of a mandated training campaign, that operators would accept, give time for and pay for - hence that authorities would actually mandate - be an effective solution?
Probably yes, but we don't know until someone turns around and says "thank God I did my upset training in that CAP10". Until then, it's best guess.
Is something more than training needed?
What do you suggest - a power-point brief? From my experience as a military QFI, you can't brief a severe upset. the student only gains confidence by experiencing the recovery.