I figured out the problem through HUACA. A more helpful agent said that as I had reserved extra-charge seats on 2 LATAM flights that were no longer part of my itinerary, they would need to cancel and refund those charges. At that point I was able to reserve new seats on the LATAM website, as it was no longer blocked. Now we will see when and if the refund comes through.

The agent said all seats would be canceled in both directions, but as of now, the seats on my return trip are still shown. So, time will tell whether I need to make another call.