Hey Yatagarasu!
Thanks for your review! 
I just want to address a few points
String thickness - Yeah, after I got a Beginner’s Shamisen, I too noticed that the hole is really thin, and had to widen the itomaki hole with a drill as well. Because the Accessory Set supplier wasn’t aware of this, I think he shipped strings a few gauges too thick. We recently informed him of this, so he’ll ship thinner strings in the set. Still, I think widening the hole is still a good idea, as it’s always good to try different string gauges.
Learning - That’s great, Yatagarasu! Please, if you’re having fun mucking around and improvising, ignore the Crash Course for now and just do that!
The goal of the Crash Course is to help anyone get started. But if you find yourself already drawn to improvisation, go for it! After all, in the beginning, the main goal is to gain comfort/connection with the shamisen, and to have as much fun as you can. If you’re finding improv to be more fun, then that’s the right direction to go in!
Later on, you can develop technique if you wish.
One big thing I appreciated about studying with Kevin 9 years ago is that he encouraged me to just learn new songs and keep going. He saw that my bachi technique wasn’t great, but unlike many traditional teachers who won’t let the student pick up the shamisen until they get proper bachi technique first (which is ridiculous, imho) he said, “Your technique will improve over time, so let’s continue learning new songs.” And that was perfect for me! Good technique naturally takes time to develop, so might as well have fun and learn more songs in the meantime! 
String Slipping - Question, is the ichi no ito flush against the left side of the tenjin, like this?
I find when the string is pressed right up against the left side of the tenjin, it almost forces the itomaki out of the hole. At least, it prevents the itomaki fitting into the hole as tightly as it could.
When I keep a millimeters of space in between the left side and the string, this problem seems to be alleviated. 
I suppose it’s a pretty decent approximation of the nagauta sound.
It’s quite close. The koma and bachi will change the tone a bit, but as always, 80% of the instrument’s tone will vary depending on the skin. If one is specifically wanting a truly authentic Nagauta tone, they would need to use cat skin (which has a higher of breaking, including the marriage.
)
Glad you are having fun with it! Can’t wait to see you jam with Kevin in the near future! 