Tumblr is a blogging platform. ManyStories is a content, blog distribution platform; they have different goals. The reason we don't want users editing the title, subtitle, and images that show on ManyStories is that we want the presentation on ManyStories to exactly match that of the contents of the story. We don't want people to arbitrarily edit this content.
If you want to update any of this information after sharing, just refresh the share page and reshare the same link. It'll pull in the new data from your story's SEO meta tags and you'll get a chance to update the tags again.
We will eventually make this process simpler so you can see a preview before you share a story and give you an easier way to refresh the contents of already shared stories. Thanks for the great feedback!