How many companies ever disclose their engagement letter with auditors? Though they are typically pretty boiler plate perhaps Mazars wouldn't want it made public.
And as far as whether it was for an audit or for quarterly review, I doubt they would take the engagement for just a review and not full blown audit as for one it probably wouldn't be worth the effort and it could cause greater exposure optically if they are not able to fully audit the numbers and just preform a review yet still have their firm associated with ONCI and something nefarious happens.
I do not think a firm such as Mazars would expose themselves to that risk to potentially tarnish their reputation, by fully auditing the numbers whether good or bad they at least protect themselves from not knowing about certain items that would not be exposed during a review.
And for those stating the whole thing is fake, I doubt Steve would use a firm that is as large and notable as Mazara to create a fake engagement as it would expose him to some serious ligitgation, if he were going to create something fake he would have used a much smaller firm that would havel limited the potential liablity