Along this line of thinking, it could work if Cirie and Parvati tell Erik they really want him to stay and Amanda to go, only they can't betray Amanda outright because they gave Amanda their word, which Amanda wisely made Cirie promise in the last ep when they were discussing the tie--that if things weren't like this Cirie would stick with Amanda and Parvati.. They say the only way they could vote out Amanda was if both Erik and Natalie were immune (impossible at this point unless a new idol is hidden, which I would call shenanigans on, or this is where the suspected dupe idol comes into play). Erik gives up necklace to rely on dupe idol (incredibly stupid, any way you slice it) and goes.

If, as the website clues imply, Natalie and Erik do initially band together, it would make sense as to why Cirie goes to such trouble to separate that unit before F4, where it only takes two votes to get Cirie out on the tiebreaker, and overlooks Amanda this round.