You have a point when saying that Backend pages page is like a CMS for the CMS, and it plays a very important role in setting backend pages properties(e.g permissions, templates etc. ) in a user friendly manner.
Changing the default URL will be OK as long as you change it just for the login page, if you do it globally for the Sitefinity group page you risk problems in styles, references to the /Sitefinity URL and I would not recommend this to you.
Concerning branding, we do not allow re branding (i.e. removal of the Sitefinity logo from backend pages), but we do allow co-branding, namely putting the logo of your company next to the Sitefinity logo.
the Telerik team