I dont know why there is such the back and forth for seats. Go to https://book-i.jal.co.jp/JLInt/dyn/a...ing/myBookings and enter the JL confirmation number which is included in your trip details on alaska site. Make sure you change the flight number from JL and the number to AS and the codeshare flight number. Enter your last name and first and then go to JAL manage booking page. Click edit seat selection and pick your seat. I have done this multiple times without issue. I have found that sometimes it can take a day or two to connect to JAL site and you may need to try your first name by itself and also try it with firstmiddle all one word depending on how reservation was passed on. But once you get in JAL.co.jp picking seats is easy.