<< Not pertinent. >>t is well known that Japan is in East Asia.
Is the noun clause "that Japan is in East Asia" functions as an adjective complement?
No, it is a complement, I believe. It modifies 'well known'. Some might call it object complement even though I do not see 'well known' as an object. See bottom of this page.So, it functions as a subject, and it is an empty subject.
Yes, a dummy subject.I don't know what you mean by 'empty' subject - would that be what I call 'dummy' subject? Yes, we do have a dummy subject in this sentence and that is 'it'.