Hmmmm, not sure if this is what I want to achieve...
On articles, I want the articles excerpt to be the meta description (which is the default behavior of this plugin AFAIK). On Pages, it should be the custom meta description, which I want to define via the function posted above.
I now filled the fields you mentioned with default meta descriptions & titles, but they are now shown on every article/page, instead of the excerpt.
What am I missing?