The Sweet covers are 95% worthless for speculation, especially for details like this.

However, in this case you've forgotten an important detail: Mat wears the medallion on his skin, under his shirt. It's not visible when he's dressed and his shirt is made up.

He could have left it to Elayne or Birgitte or have hidden it (he may be afraid the Finns could steal it during the rescue), but the cover art doesn't help one bit one way or another. Between the gholam and other potential pursuers (if it's anything like Perrin...), Mat also has plenty of very good reasons to wear the medallion at all time and risk bringing it into Ghenjei. In the middle of nowhere like this, he needs all the protection he can get from channellers/gholam and so on.

It's not a bad call about the dagger, though. Not at all. If Mat is hunted down by gholam, Forsaken etc., I could see Elayne giving him the blunt dagger for protection (which he would try to refuse, insisting that it's her the gholam tried to kill, but she will insist). That could well save Mat's life at some point, and there's foreshadowing Elayne might.

I think you're really on to something, with the dagger being in Elayne's hands, and Mat coming to her, and the gholam still chasing Mat (and finding Elayne, eventually too). You may very well have found out the real purpose of the dagger. Not something for Rand in the Pit of Doom or to hide from the Shadow he still lives (this always seemed to me way too obvious), but merely a plot device to hide from the gholam, and maybe sneak up on it. Whether it saves Elayne's life or Mat's remains to be seen, though. It could really be either of them, as both are targets the gholam is still compelled to hunt down.