I'd like to create several sitemaps and gather them into one sitemap index. Then I'd like to submit the sitemap index to Google (and other SE's) using those links. From my point of view it should be enough to be well indexed, but I'd like to be sure that it is not necessary to point the sitemap index to Google's Webmaster Tools.
You don't have to use GWT to make Google aware of an XML sitemap. It's just one of the ways they make available to webmasters but isn't the only way or a required way. You can use robots.txt to accomplish this as well.