If you're using that cord for a string then you'd need more than just the one strand. Remember, a bow string needs to have a breaking strength of AT LEAST 4x the draw weight. For example, if the bow has a 40# draw, then the string should have a breaking strength of 160#. Because your cord is 70# breaking strength, if you make a braid of 3 then it should be fine.
The backing depends on the grain of the board. Hickory can generally have poorish grain and still be ok but there's always a limit. In order for us to judge the need for a backing, we'd have to see the board and its grain.