I'm sure that there are more experienced members who can answer your questions, but here are my personal observations:
1. I fly the YYZ-South East Asia route somewhat often (3-5 times annually), preferably on SQ (with AC connections), sometimes on AC, sometimes on CX, sometimes on KE. I must say that the service I've received on AC (albeit in business class but I can't imagine it being that bad in economy) have been nothing short of courteous, genuine, and professional (although not in an overly indulging manner like that you would get from Asian carriers) and certainly far from rude. Overall I'd say if you didn't come on board expecting the royal treatment you would not be disappointed. I have no experience with the seats on CX but the AC seats are not bad although I personally think they could use more padding. CX's IFE of course is far superior and more reliable (which might compensate for the hard seats
)
2. There are student fares on CX (not sure if they have it ex-YYZ), but they aren't necessarily the cheapest, they just entitle you to extra baggage allowance and more flexibility in some cases.
3. May-August I believe is shoulder season, $1356 on CX quite good, I believe last year a family friend paid around $1200 for YYZ-CGK in April. Are there change of date fees involved with this fare? Not having change of date fees may be handy if your plans are not yet firm. The change fees on the cheaper AC tickets will milk you dry!
4. I'm not sure as I have always bought my tickets online and I don't hope for much discounts since my travels are usually during peak periods but I'd imagine some agencies might have access to promotional fares that you would not be able to find online.
5. Not that I'm aware of.
6. I believe the next higher fares will get you 6 months stay. Not sure how much more you'd have to pay.
7. Definitely, just make sure you get your onward boarding pass from the transfer counters before you proceed to passport control.
Just to throw in an alternative for you, EVA Air might have competitive fares ex YYZ although they only fly thrice weekly, I'm also not quite sure how the schedule is for SGN connections.
I hope this helps
