'Onward travel' confusion

Whilst researching an upcoming big trip I have found out that many countries will not let you in if you don't have a return ticket or 'proof of onward travel'. I am now a bit stuck as to how this all works, and am hoping travelbloggers can advise!

I plan to fly from London to Buenos Aires and a year later fly out from Mexico City to London. In the meantime I travel around South and Central America overland. I am going to book both flights with British Airways, and the return flight will be booked to come back within 90 days of my flight to B Aires. (I think this should get me around the 90 day visa limit in Argentina, and for a small fee I can change my flight to come back from Mexico later when I am in South America). But, I have been advised by a travel agent that because Mexico is on a different continent to Argentina, the Argentinian authorities will not consider my flight from Mexico to London as proof of onward travel (although personally I think it is pretty obvious I will have to leave Argentina to get to Mexico, but that is just me!). Has anyone else gone through these problems? (either in South America or elsewhere in the world). What do I do?!

Thanks for any help you can offer, Ruth Reply to this