Sentence Correction (SC) | Re: The fact of some fraternal twins resembling each other greatly and

KarishmaB GMATNinja

That some fraternal twins resemble each other greatly while others look quite dissimilar highlights an interesting and often overlooked feature of fraternal-twin pairs, namely that they vary considerably

Can you please help me understand the sentence structure?

A) That xxx (Sub) highlights (v) - I believe sentence starts with "that, which when" etc is a noun clause and "that +verb" is a DC and not IC.
Ex - Whoever thought of that idea is a genius. - subject - noun clause- Whoever thought of that idea

Even if we consider "That..." as IC, what is the role of "namely that they vary", Is is a DC or IC?
I think we need FANBOYS to connect IC to a DC.
B) Can we connect using that? - "IC, that IC"

Thank you forhelp!