hi.

is secondhand handbrake cables ok to use if the 2ndhand ones good

where the handbrake cable runs from the leaver under the driver front door to the park brake arm does the cable get srapped to anything beides the leaver and how easy is it to slide it through the firewall

My quick answer is NO. I would NEVER use a second hand cable nor brake parts. The cable may be stretched and not so detectable.