Dear Mike,
Here's just a few initial responses to your questions/statements based on my understandings and experience....

: I want to draw your attention to the following points: 1) stern section
: in the picture just above that statement (page 3 top-right) you notice a
: clearance between the horizontal stern plates, aft deck stringer and the
: stern piece - more detail of the same on page 6, picture in 2nd row
: 2)cockpit - front side
: in picture page 6 bottom right you notice a clearance between the front deck
: stringer and the masik, accompanied by a similar clearance on the part of
: the stringer that dives beneath the cockpit coaming

-- These gaps might be explained by the fact that this is an old dried out frame. They'd certainky be tighter when the frame is enveloped in a skin. That said, I agree that they are formed to allow movement.

: 4) dowels
: in the pictures quoted above we can see that both front and stern plates are
: doweled to the gunwales by only 1 dowel

There are actually two dowels on each side of each horizontal plate-- the photo has been cropped. (Note lower left image on page 27).

: In none of the modern baidarka building descriptions I found sofar any
: mention about such clearances. They also frequently tell you to use 2
: dowels for the front and stern plates which would efficiently prevent the
: flex that was said to be a major advantage of baidarkas.

A single dowel would not encourage more flex so much as it would encourage the dowel to break. The cross pieces being severed still allow for considerable flex.

: Coming back to Zimmerly's statement about "exactly copying internal
: structures" my preliminary opinion is that this statement is wrong.

It all depends on what one's goal is with building a "baidarka." If you are after the 'secrets' of their flexibility, it is best to build as close tot he original as one can. If you are after the shape, you could even build it out of plywood, cedar strip or ferro-cement. I personally believe that analyzing the hull-shape without considering the intricacies of the structure and joinery is wrought with error, but it doesn't necessarily mean you'd end up with a bad kayak.

: What is your judgement about the above: Has this sofar been overlooked /
: neglected / found insignificant / am I misunderstanding something
: fundamentally here)

The best writings on these matters comes in "Contributions to Kayak Studies" published in 1991 by the Canadian Museum of Civilization (Edited by Eugene Arima). It may be worth getting your hands on a copy-- fascinating stuff by a variety of authors.

: Thanks for a good discussion and
: Kind regards from Austria
: Mike

Thank you, and all the best,
Harvey