1) Does the Whitepaper become a commercial offer? or should I have a separate agreement for the token sale?
What does that even mean ? If you're asking,yes you need to have a whitepaper for people to know what your idea is all about and most importantly you're serious about the project.
2) How many tokens should my company leave for my own use (in percentage)?
That you have to decide for yourself man.Why they should though ? Will you be the lead developer/founder?
3) Do I have to buy back all tokens that came out to the trade or can I buy back only a certain amount of tokens?
Reserve them I guess.
4) Should I start selling tokens with a bounty during the pre-ICO?
Again,depends what works for you.Selling before the ICO is open is not a good idea,bounty tokens are okay.
5) What is the best place to establish a company (legal means)?
Thanks!
How old are you ? It literally has to be in your country according to your business laws whatever.