Understanding the splices in AC43.13

I’m trying to understand Figures 4.45, and have a question about a
technique shown in 4.44.

First, 4.44 – here they show fabricating an inner tube by cutting,
reforming and welding outer tube material. Is there any reason this
can’t be done with round tubing if one can’t locate the correct size
inner tubing for an internal splice?

Second, 4.45 – are they showing cutting slots in the outer tubing?
In the lower drawing what is that streamline shape illustrating?

Thanks for your time - Mike