This is a long standing gripe and pain point unfortunately. I take icelandair often and if booked via Alaska's website as an AS codeshare neither AS or FI will allow you to choose your seat until 24 hours prior to dept. I've corresponded with some people high up at Alaska and they know about it and are apparently working on a solution but I'm not holding my breath on that happening any time soon. If you book directly with icelandair you get less miles but you can choose your seat and change it until your heart is content.