1) For a variety of reasons including that founders tend to have stock while employees tend to have options. Also the more practical reason that founders tend to be closer socially means that they're somewhat less likely to screw each other over.

2) Depends on your jurisdiction. If your company engages in illegal activities and you're a major shareholder/director you could be liable.

3) Share of equity, control.

4) Not really, it's just a matter of agreement.

5/6) Normally as a founder you trade-off pay for equity. If you're coming in as an employee post-fundraising there's obviously less risk involved as well.