You can try ticketing with AA Canada if you're in Canada between now and the commencement of your itinerary. If you have at least one transoceanic segment on AA, try ticketing via a Korean or Canadian TA.

Alternatively, if you're brave, you could get the ATW desk to set it up for you, pay the AA GSA in Seoul and issue the ticket upon arrival in Seoul. Unfortunately you have to be physically present to pay the AA GSA by credit card and they do not accept international wire transfers.

I usually get the ticketing carrier, ie AA, or TA (whichever channel I'd used to issue my ticket) to do seat assignments for me at time of ticketing.

Whenever I've changed my itinerary after departure I've usually done it with the ATW desk and so have also asked them to change my seats in the same phone call.

For adhoc seat changes without involving an itinerary change the operating carrier is often happy to change my seats for me.

For AA and JL segments you'd be able to do it on their segments with PNRs for the respective carriers.

If booked with AA, you'd be able to see the entire itinerary on aa.com. If booked via a TA, ask the TA which reservation system they'd used for your itinerary and its PNR on that system. If the TA is ticketing on AA document, more often than not you'd be able to see the whole itinerary on aa.com after the ticket has been issued - however you might still want to refer back to the "native" GDS.