When you select the Post Excerpts option, the only change is in the RSS feeds of this blog. It will not impact widgets or tiles, unless it is the RSS widget/tile.
In regards to your second question, a group blog page can be accessed from the group content page, on the left-side of the page under View the Blog, as shown below:
Hope this helps.
Thanks, that answers my question.
Is there a way to change how they appear in the widgets/tiles? I'd like a more expected blog experience, with an image, blog title, beginning of blog, click here to read more, etc.