Review and Comments
Well what to say - it is a valid DONE4, that presumably goes to the places you want to go to
The only comment I have is that if you are looking to maximise flying/EQ Miles and/or dollars/AAward miles then perhaps change SEA-LAX-NRT to SEA-DFW/ORD/JFK-NRT
Your question
If you buy online, then you will be charged in ZAR and your ticket will be issued by IB (I assume). For subsequent changes, you will deal with IB. Some credit cards charge a fee for transactions in foreign currencies.
Provided you have at least one "over water" flight with AA then you can buy the ticket from them
You can tell AA that you want to pay in the the US. AA will convert the all-in ZAR price to USD and charge you that. For subsequent changes, you will deal with AA. My current DONE is AA-issued; I have done a re-route and have made three date changes so far; all have been totally painless - just phone the ATW desk and it gets done. I have never dealt with IB so cannot comment there.
The AA price will likely be different from the online (IB) price. This is because different airlines are more or less zealous in collecting other airlines' surcharges.