Newbie with build questions

Hi everyone, I’m Chris. I’ve been lurking here for around a year or so and decided it was time to introduce myself. I don’t have a shamisen at the moment, but I did get some Bloodwood and African Padauk today, so I plan to start building a Tsugaru Shamisen soon and have a couple questions that I’m hoping someone can help me out with.

  1. I read in Shamisen of Japan that the top of the sao wasn’t perfectly flat; instead, it has a shallow curve both lengthwise and widthwise instead. How important is this and can I ignore it so I can finish my shamisen a little quicker?

  2. What purpose does the rindou on the nakago/ring on the dou serve? Is there a specific reason for it, or is it purely aesthetic?

  3. I know Shamisen of Japan recommends African Ebony for the itomaki, but would a softer wood be more ideal? I’m asking this primarily out of concern for possible excessive wear on the tenjin as opposed to the more-easily-replaceable itomaki, but also partly as an excuse to use Pale Moon Ebony (Janka Hardness of ~1780) instead.

  4. Is the thick calfskin from mid-east.com really OK? I know it’s recommended in Shamisen of Japan as well as by others on Bachido who have used it, but it’s half the thickness of what’s recommended (1mm) in the Kawahari section of SOJ and I’m a little paranoid about it.