Except he WAS right about her feelings for Rand and he WAS right about that being her motive for speaking to Elayne! Even if he does recall Aiel polygamy, it was presented to him as a choice women who are friendly make to stay friends (Rand never heard of it before Rhuarc introduces Amys & Lian as his wives and after they explain, that is the impression he has of their threesome - that their relationship came first and took precedence over their marriage to their husband. That would have been Mat's introduction to Aiel polygamy as well), and is still not the intuitive leap he could be expected to make. If you were to encounter two Middle Eastern women who liked the same man would YOU expect them to get along, knowing that the Koran permits a man to have up to four wives? Or if a girl from Utah was going to visit her boyfriend's girlfriend, would you assume she was going to propose a polygamistic relationship? NOT EVEN AVIENDHA had that in mind!!! She was surprised when Elayne hinted at it! Mat anticipates trouble when he realizes how she feels about Rand, but anyone would if she NEVER drew a knife. Even Siuan suggests as much about Elayne & Min back in the prologue, saying if they are going to fight (using terms like "hair-pulling" which plainly denotes physical conflict, and at the same time lacks any connotation of hyperbole - suggesting she would not be surprised in the least to see them come to blows), better they get it out of their system in Salidar rather than freaking Rand out. And once she pulls out the knife, there is no way ANYONE could think she meant ANYTHING other than to use it on Elayne.

Oh, yes. The concept of biding your time to strike has never ever crossed Mat's mind. Aviendha is travelling with Aes Sedai, and Mat has not seen her with fewer than two sisters since arriving in Salidar. For all he knows, she is being made to play nice because they are trying to recruit her or something. He has no reason to believe she has mended fences with Elayne or that she has given over wanting to kill her. Neither of them has given up on Rand, as we the readers are well aware, and why would Mat think Elayne would go along with sharing him? Elayne herself doesn't think she would back in EotW and still has doubts as of tFoH. In fact, she first becomes fixated on bonding Rand as a warder as compromise with the inevitability of having to share him - she thinks that if she can't solely have him, at least she'll have his bond for herself alone. Her sharing the bond, devising the means to split to include a non-channeler, and generally being the glue of the foursome (not to mention most likely source of financial support in the future) are marks of her extraordinary generosity and willingness to go the extra mile for people she cares about (to a ridiculous extent, IMO, but that's neither here nor there). To assume or take all this for granted on Mat's part would be a far more egregious presumption than what he actually does believe.