Just to post my experience this week- I was able to select non-throne seats prior to 48 hours before checkin. Throne seats could only be purchased at this time since I'm not SEN.
Then at around T-48 I could not select or change seats. In some cases I was able to select but it never saved, and in others I got an immediate error saying it wasn't allowed.
I was only able to check in at T-22.5 (not 23 as the website said or 24 hrs as is common) but at this point I could select a seat and was able to get a throne, 4A. Not the best user experience but happy with the outcome. Unless I should have chosen 5K... ugh.