Although certain parts of the Twain house are undergoing restoration
right now (primarily, I think, the downstairs library), as far as I know
open for tours. As someone doing some serious research, you might call ahead
to see if you can get someone on staff or a top-notch docent to take you
through. Call (860) 247-0998 between 9 and 5. John Boyer is the director,
he's a good, smart guy.
Kevin B. could probably help you with the New York end. I know the
forum did a birthday walking tour last November. Good luck!