If you bought EBCI for four people and left your wife to the regular check-in process, other than not standing together in line, I think it would be fine to hold a spot for her when you boarded. So either buying it now for her and then hopefully Southwest can transfer it if you did need to rebook (or maybe they can't and it would be lost), or buying it for her separately later, for a position that won't be as good as yours, but still before the general check-ins, or not buying it for her at all - in any of these scenarios I think she'll end up sitting in the same seat.
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