1-only clan leader can accept to change his faction for the whole clan.
2-you need more charm to succeed.
3-You also need to look at your counterpart in the encyclopedia before to better known him/her. If you see (under the portrait) honor / mercy / generosity... it will help you to choose appropriate answers.
4-You can't try again (or need to wait several game days). The best you can do is saving, trying and reloading + re trying if needed.
Hope this helps