Is seatpost length quoted just for the tube section, or does it include the saddle clamp?

Also apart from i presume rider height, what governs if to go for a 367 or 410 Thomson – does more seatpost length within the downtube = greater strength?

usually from rails to the bottom. Not alwyas though so do watch out.

More seat tube in the frame (beyond 1″ below the bottom of the seat tube/top tube junction) should make no difference though it’ll depend on the specific frame design to some extent.