1. Is it possible to change the name ".../cb-profile/..." to another one like ".../community/..." ?

Yup, that's the alias for your Joomla menu item to CBs profile page. You're welcome to change the alias to whatever you like.

2. In the profile the user sees own profile url like www.domain.com/cb-profile/154854-username . If the user wants to change the url to www.domain.com/cb-profile/username , cb says that the url is already in use. But it leads to own profile, too? So both ways go to the profile, but no way to show the better url (without numbers) in the profile to other members?

Their username is not alias safe. That means when it's converted to alias safe using Joomlas alias function it no longer matches what their username is. This results in the user id being prefixed otherwise it will not be possible to properly match them in the database query to find their user id. Their profile url can not match anyone's username (including their own) or anyone else profile url.