I would either use the title or a separate field for the headline (although I don’t quite understand why you would need 2 fields (the title plus a headline field), because you cannot trust that the first heading in a blocks field is what you want as the title of the article.
A type teaser for the blocks is a bit weird, because you only want one teaser per blocks field.
In my opinion the block structure should be closed. It is a complete article with headline, text … sub headline … an so on.
You are right again, a own teaser definition is not necessary, because the first paragraph should be the teaser. It was a careless quick shot. (thanks to my brain for the creative phase )
If you only do this for yourself, then this could work. Editors usually need to be forced into some structure, or they start such a block with text or the wrong headline type or whatever. So I see the blocks as longform text without title. And as I said, a teaser could still be a separate field for more control. But of course, it’s totally up to you how you set up your information model.
Yes, you are right. There are so many ways… Now I tested your first idea and it runs. Now, my panel is very small and I have all the information redundancy free. This is very nice.