Would japanese yen fit in it? (6.3" x 3") i don't want the yen to fold.

When I was in Japan, I had no problem keeping Yen in a wallet of these dimensions and keeping them uncreased

Can 2 cards be put per slot or are they too tight?

At first the slots are extremely tight, but after a while they loosen up enough to put two cards in a few of the slots.

Does this wallet have a zipper pocket? One review stated that there was one but none of the pictures show it.

Each side of the wallet has a pocket. On the left side (as I hold it) there is a zippered pocket inside the larger pocket. Large enough to hold s few folded bills and still remain inconspicuous. The left pocket also has "accordion" folds around three sides so it can expand to hold multiple bills. The pocket on the right is "flat", that is, it is sewn around three sides, limiting the number of bills it could hold. As I describe the billfold, the left and right i indicate is with the window on the right side. Both sides have additional pockets to hold credit cards. A total of 12 cards could be handled, without doubling up on a pocket, including the window pocket.