I think what happened to Lindsay happened when she was a teenager, or maybe in the first years of when she was in college. Because the way she talked about it in "Silent Night" sounded like it happened a while ago, and Lindsay didn't know why these memories were cropping up now.
It sort of reminded me of that episode of "Criminal Minds" where those three girls from the soccer team were kidnapped and the unsub kept them locked in a basement, and he said only two of them would come out alive.
I expect that the violence of Lindsay's secret was probably some intruder at a sleepover or study group. Multiple stab wounds, blunt force trauma... they never found out who did it, and that's why she became a CSI. The need to find justice, you know?
I think Mac still would've hired her. He obviously cares about his employees, and any struggles Lindsay went through he would've gladly helped her through. He wouldn't have hired her anyways if he thought she couldn't handle it.