I watched the episode twice and didn't get it.
Watched the scene twice after Xant's post and to me it seems that Olenna is removing one of these blueish crystalys from Sansa's necklace when she fondles her hair. When the two are joined by Tyrion there are only 6 of them to be seen (while there are seven on the previous episode when former Ser Dontos gives the necklace to Sansa) and one of the golden triangles which hold the crystals is empty now. How the poison is spilled into the wine however is still a mystery to me.
About Benjen (not a spoiler as this is again different in the books): in the books Sam and Gilly are being lead to the secret passage under the wall by a mysterious zombie Nightswatch ranger. I've read theories that this is Benjen Stark though I do not believe this.