I had mixed reviews with Japanese auction site - one item arrived with good shipping, the other item did not and had freight shipping I believe. It was not a big deal to me but I would give them a low pass because there are no other options. This may be because of corvid.
2) I have 2 of the 3 Japanese keys mapped to control/modifier and and the third mapped to space. Your picture seems to be off there aren't four modifier keys but three. The switch is quit seamless between a Mac and Windows. Medium Hands. I was analyzing keyboard layouts.
No the Lenovo ThinkPad Compact USB Keyboard with TrackPoint(I don't mind the differences so much.) Using SharpKeys this is really easy. I do have a traditional Thinkpad keyboard non Japanse though.
All you do is press the key in the "Type Key" blank in SharpKeys.
I have actually gotten a Japanese Thinkpad keyboard for my laptop although its for the T420 model. I would have to check my Desktop to see how this works again(on a laptop right now) if you want the actual scan codes. I will say I haven't gotten this to work on Linux just Mac and Windows unfortunately.
[1]: https://www.medscape.com/viewarticle/948449