About the proof of onward travel and such. I faced a similar issue in September as Continental would not let me book a one way nor book a flight beyond 90days.
I posted to this board and got several responses (maybe do a search) the recap of is this:
Yes, it is apparently true that Argentina does not require onward travel proof, but the airlines can set their own rules as they are the ones responsible to fly you back in the event you are denied entry. (I guess if immigrations asks you your plans and you have no return ticket they may think you are going to be an illegal alien....). So the airlines can deny your purchase.
Along with buying a fully refundable LAN ticket, someone suggested a buquebus ticket that is refundable or good for a year since you may likely use it at some point during the year (whether for Visa runs or just to go to the beach).
All this being said, like everything at these big call center type companies, it all seems to depend on who you get on the phone, if they know their job, what kind of mood they are in, and how charming you can be on the phone. If it was truly policy this message board would die as everyone would have the same story.
I have to buy tickets soon too, so I will let you know my experience.