let me try to answer to your questions:
OK, Blasius, I looked at your steering video today. Very nice! What is the secondary steering line doing while you are using the normal handle? I see you put it through the "rollbar," but I can't tell what it's doing as the regular handle is moved. I has to be moving as well.
Yes the secondary steering line, when not in use. MUST stay so that it can freely move, without any risk that it can get stuck when you move the handle. I tried different solutions, and the one that seems to work better is the one you can see in the video, where the line move inside a piece of pipe (yellow one) that I put in an hanging position when not in use. The second pipe (black one) that is connected to the previuos one, as you can see in the video, is used in order to block the yellow pipe in different positions when in use, so that I can use it as an handle. As you can understand depending on the side you are hiking off, you need to pull the line from a different position.
Also, do you have to keep pulling the secondary steering lines taut (by keeping your arm pressing towards the bow of the boat) while you are using it?
The secondary line is able to steer in a very smooth way, there is no reason to keep it taut, unless you need/want a very fast steering reaction. In normal wind conditions I use to keep it slack and tighten only when needed.
Also it looks like I am seeing combination backrest and seat in the second video. Also very nice.
There are different reasons because I made and prefer the wooden backrest to the original hobie seat.
The main one is that the original one tends to fold under my ass when I am pedalling.
After that I made it and tried, I realized that it improves and adds functionalities (e.g. Rollbar, it can be used as a raised seat, it can be used to hang the secondary steering line and so on), and expecially it is much more confortable for my back than the original seat.
The one you can see in the video has recently been modified with an additional lumbar support because of my hypolordosis problem.
In the next days I will publish a new video where you can see the last mods.
I am very happy with it, and I will never turn back to the original seat.