Oleg, I just looked at your video based on your last comment.
If you mean the plank as a seat, it's not a structural element and they work without them.
I think your materials were a bit heavy. Older un-coppiced timber has a mind of its own. The band of gunwale weaving wasn't strong enough to deal with the ribs trying to return to their original shape so when you took the frame out of the ground it flattened.
There is more to forming the frame then just bending the branches over and tying them to each other. They will try return to straight. You need to 'educate' them by breaking the fibres along their length. If you look a the old footage the man bends the hazel 'in' his fist, a fist at a time, from base to tip. It is most important at the heavier base end. I knew this when we came to make the coracle because I've made willow fencing. (It's easy to forget you know stuff when you just do it without thinking.) I think Bill 'just knew' as well.
We left the weights on the frame for several days also.
As for using it; We don't sit up in ours so much. We kneel in and just perch on the seat. This lowers your centre of gravity and having your knees and feet on the floor gives you 4 points of contact which gives you better balance. This also puts you close to the water and nearer to the edge of the gunwale so is easier on your arms and shoulders when paddling.
There you go, all our secrets are out