SpencoSpenco Total Support Original Insole, Women's 11-12.5/Men's 10-11.5

I wear 8 1/2 shoe & ordered Women's 9/10. They are too long & too wide. I could trim length, but not width. I'm afraid the size 7/8 would be too wide. I don't understand the sizing.

Are these good insoles for someone with fallen arches and can they be used in athletic training tennis shoes?

Will these inserts work in women's flats?

Does it come in pairs or one piece with this purchase?