I'll try and answer some of these, but others may correct me if I steer you wrong:

No problem to visit South America with a OW circle pacific, but that locks you into the longest and and most expensive ticket (<29,000 miles even if you don't use them all). You should search the Evil Empire forum too, since for the reason below you may get more joy with their circle pacific product. The basic idea for both is that you cross the Pacific once to the South in one direction, and once to the North in the other. The gateway cities for the northern routing are HKG and NRT (CX and AA respectively).

The problem you will face is that the Circle Pac. ticket is only good on OW metal, and getting OW metal to South Pacific islands is kind of hard. QF only runs codeshares to Apia and Nadi, but I think they have one flight of their own metal to Noumea (from BNE), so that may be an option, although only as an out-and-back from Australia. To get to South America, you'd need to get QF to AKL and then Lan over to Santiago. The other tropical island you can of course visit is Hawaii, via SYD or the continental US.

This is called throw-away ticketing, and is prohibited by the airlines. The penalty for doing this is that you run the risk of the airlines repricing the ticket as a series of one-ways. Not good. That said, people obviously do it so the risk is yours; however some friendly advice would be not to run around publicising your plans on this front too extensively if you decide to do it.

Well you must by definition get off at one airport and on at another, so overland mileage is from the airport you left to the airport you next board.

You might want to research the rules here and think about your routing some more. The links above will help with ideas too.