I encourage you to do what you think is best, I'm only giving recommendations based on my experiences. I've been using these SOF boat exclusively for the last 3 or 4 years with no joint failures. I regularly run a shallow river near my house and have hit rocks going from speed to a dead stop with no issues.

I would definately not use screws as the only method of fastening. I'd worry that overtime the continual flexing would open up the hole and the joint would fail. The failure maybe catastrophic, a failure with a sinew join is most likely just sliding in the joint. Stainless screws are expensive. My first boat had some screws the joints seem OK but I lashed over them.

Lashing would take forever if you had to tread it through holes in the stringers. If you pull the lashing tight enough there will be no bumps in the skin.

I'm not sure what the test really proves. The test loaded the joint in compression with a little bending caused by the impact offset from the form. The compression loads could be handled entirely by the form wether it was fastened or not. The bending moments in a complete kayak would be resisted by the skin (only marginally by the joint).

There are other sites that cater to Greenland kayaks that have a very deep SOF following, you might want to post there as well to get some more opinions.