I think that you should forget everything except the plans sent to you by Rob Mack. As many people have built the boat with little problem, I would think that problem is in your understanding of the plans.

Stop everything you are doing, start all over at step one and use Rob's way on everything. Combining different approaches and building techniques is probably not a good way to build your first kayak.

Rob Mack would have trouble advising you on problems if you are using Nick's method to build. In fact I would say that it might be impossible for him to do this.

I have seen many pictures of the North Star all build Rob's way and they don't seem to have the problems that you are experiencing. Check your own practices before trying to modify the established building technique.

Hope this will help, Ian