Originally Posted by Often1

Are you certain that this is a fare which has no credit as oppose to refund value? Typically UA domestic non-refundables purchased in the USA can be cancelled prior to travel for a credit less change fee. That credit can then be applied to a new ticket within the next year. I do appreciate that there are consolidator and other bulk purchases for which there might be no credit value, but you should certainly verify all of this first.

As to blocking, it's not your seat, but the non-flying individual's seat and if they don't board, it's empty. While there is always a chance a GA might block the seat, they certainly won't do that unless there are empty seats and no pax waiting for them. Given that this is Hawaii over the holidays, greater likelihood is that the flight is oversold.