Here's a thread by someone who bought their Kobo in Japan and wanted to use it in English elsewhere... It appears to answer several of your questions (in the inverse but it does give insight as to how the Japanese site works):
1- use the Rakuten Japan site to register to Rakuten and then register the device with Japanese credentials
2- yes you need to change the IP address so you appear to be in Japan
3- I would guess that Rakuten may have some on their site similar to Kobo on theirs?
Perhaps someone else with personal experience may be able to give you more help... Good luck!
https://www.mobileread.com/forums/sho...en#post2345976