I'm no expert on this subject by any means, so I'll gladly yield to someone with more detailed knowledge.
But my understanding is: Publicly traded companies are required to disclose accounting-type information to the Securities and Exchange Commission and to shareholders. The kinds of things they have to report are your classic accounting categories: income, expenses, assets, liabilities, capital. They also have to report the names of certain top managers and how much they are paid. See, for example, http://www.inc.com/encyclopedia/sec-disclosure-laws-and-regulations.html Most of this should be in the annual report. If you are a stockholder, they should send you one automatically every year. If not, you can contact the company. Some companies post annual reports on their web sites. There are services that will supply annual reports, for example, http://www.nasdaq.com/services/annual-reports.aspx.
Most companies include total number of current employees in their annual report somewhere. I don't think that's required, but most do.
But to the best of my knowledge, none of the information you list in your question is required to be disclosed. "Why they left" would be almost impossible, as the company as no way to know for sure what was in someone's head when they turned in their resignation. It's common advice to tell people that when they quit a job, they should just say that they are leaving "for a better opportunity" or "for personal reasons", and not to say that it is because the boss is a jerk or company policies are stupid or otherwise complain about the company.