mumko, Welcome to FT
1. Given Mexico is a ticketing location (I think it wold be included in USA, Canada, Mexico grouping). I presume the wording is more to prevent people gaining mileage from backtracking given LAN does not fly to any of those central/ caribbean locations. Note Mexicana is a recent entry to one world.
2. I doubt there is any tolerance on the mileage limits (I suspect it is pretty hard to go over 29k in any event)
From Seoul would probably buy direct from AA (American Airlines) - either the Korean agent, or the RTW desk
http://www.aa.com/i18n/aboutUs/where...kets/korea.jsp