I have only found 6 keys as well. I had done almost all of the side dungeons before Elemental Abyss and gotten Darius and Vahn's mother weapons. I chose:
- The dagger first because Laka had nothing and it was a huge boost (3 other users as well)
- The staff because 4 characters can use it
- The Sword (Mythraven) for Lyric
- The Darksword because it has higher Vigor than Darius' mom-weapon
- The ninja blade has 200 spirit
- The lance since you have 2 lance users (it's great for Adder)
SAVE before choosing and pick what you need most. On the other 4:
You get a 900 vigor axe from Brindle if you have him as the blacksmith in your camp after the elemental fiends are defeated.
The bow is meh.
The rod is also meh.
You get better hammers for Cid and it has no magic for Vera