1. If you don't kill the Overseers on Tatooine, they will take Hulas away and that's the last you'll see of them.
2. Any Overseers you kill won't appear on Tatooine and the rest will. No other difference.
3. Yeah and I tried my best to work around that. For example, with Vorn you actually have to fight two Genoharadan spies who find out you let Vorn live and then lie to Hulas saying you were all attacked by Sand People (you need a high persuade skill for that, or Hulas will know you're lying and you'll be kicked out of the Genoharadan).