Which of course mean the Finns had to have some way to neutralize them.

There's no medallion like the one they gave Mat in any of their descriptions.

Which ter'angreal cache?

Not the Tower, entering Sindhol that way isn't covered by the treaties and extremely high risk. Mat's luck is what made him reach the Chamber of Bonds safely.

Moridin must have used the Tear Doorway. And yes he made a bargain. It's fairly obvious from the way the Finns brought him around to show him their female prisoners so he could choose. Moraine said he looked at her and told the Eelfinn guide she wasn't the one he wanted.

Looking at the way the Eelfinn went completely nut devouring Mat's eye, I think it's fairly obvious how Moridin paid.

- "Hello, what will it be for you today good sir?"
- "I want to see all humans you hold captive and buy the one of my choice whom I will kill."
- "You're a bit weird, but well, we've got weirder. And for your second boon"
- "You still make dreamspikes? I'll want one, functional, and with the proper key to it"
- "Problem with the tax collector or an invasive mother in law? Poor you. What will be the third boon?"
- "Hmmm.. you'll make me a second dreamspike with the proper key to it."
- "Wow, this is gonna cost you an arm and a leg."
- "Oh, you'll have that and twice over. I'll give you the body of the woman after I kill her".
- "Oh, yummy. You Chosen are always fun".