There are lots of plugin available for that, use them to rename user_login in your site.

Its free 🙂

Check in AnsPress options -> permission

Yes. Just add [anspress] to the page you want this Q/A forum to appears. This is independant of the rest of your website and once logged, your visitors will be able to use it. This is a plugin and not a theme, so you can use it with the theme you want.

Thank You Rahul (it give me an idea for future update, when typing your name, let the system complete it and link it to your profil or like twitter if we could use @Raluh_Aryan for exemple)

This fix is on the last plugin version ? I have the same issue still today.