The plan was to have the 757 flights operated by SNN crew, including the DUB-YYZ. The A330 is 8 cabin crew - minumum crew levels for that aircraft, so Im not really sure what the argument is for 5 on the 757. Safetly has not ben mentioned, so I don't think that is the problem.
I would have thought that with 5 crew there would have been two crew looking after business, one in the galley one doing the service and helping in Y with tea and coffee and three in the main cabin. I'm not sure how the current J service could be delivered in a timely manner to even 12 seats, unless they plan on having just 2 crew look after Y?