I'm not familiar with ThemePen, his posts, or his merit-giving history other than what was shown here but I agree. I'd say every local board that has a sizeable number of active members needs at least one dedicated merit source--and I've been of that opinion for years now. It allows members who might not be able to write in English the chance to earn merits more easily and prevents the main section from being flooded with incomprehensible posts.
I think basically the same.
I think most campaigns pay for posts from local boards, right? So that's another reason for local board merit sources.
Yes, today most pay for at least part of the weekly required posts on local boards, and some pay for posts on local boards without limit. So, as you say, local merit sources can also encourage people who are not fluent in English to create constructive content and they can also get paid for it in their local section.