I don't know of any interviews or other material that explains the writers' motives for bringing back the First.
I assume so, Amends basically introduced the Powers That Be, and the First is basically Satan, So I assume the episode was intended to be a bare bones introduction of the Angel The Series Mythology, and they intended to return to the character at some point.
However, I doubt anyone had a master plan that included season seven as we know it. If Buffy was a novel, yes, definitely. But as a TV show? No.