Well, personally, I would move on, try to find a new job, IMO you would be wasting your time trying to sue her, you knew the company rules, you knowlingly violated them.
You have an uphill battle as it is, you had a joint acccount with him, thus he had full access to it, at best, he would be liable for half of the overdraft, the car, well, even if you get a judgement, you will have to be able to collect on it...not an easy task.
I agree, it is a real mess, one that I hope you will avoid in the future. If you haven't done so, I would recommend that you read Codependent No More by Melodie Beattie.
It is not so much the circumstances we are involved in...it's how we handle them.