All of the a/c lines on my mother's focus are "pitted" all over, leaking and need to be replaced. The valve core seems to be in good shape. Should I replace it anyway? I am definitely replacing the accumulator because its line is pitted too.

Hello, yes, I personally would replace all the lines and the valve core, and any component associated with the a/c system. To see all the a/c components we have for your 11' Ford Focus, please see this link https://www.carid.com/2011-ford-focus-air-conditioning-heating-parts/?filter=1&fitment_type%5B%5D=Vehicle+Specific