I am not sure that you understood how the Yoast SEO plugin works, so let me explain some things.
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The “focus keyword” is just some feature to “test” your article, regarding how well it would/could probably work with the focus keyword. This has no effect whatsoever to the outside of your blog. It´s not like Google sees that focus keyword and responds to it.
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The structure of your “SEO title” can be changed in the plugin options, no need to put some crazy filters on this.
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Your meta description can be easily added directly in the post and is saved automatically together with your post. No need for filters here, too.