Your claim about no issues with liquidity are outright wrong. Anything beyond the top 100 names in section1 you will deal with liquidity issues. And shorting Japanese names? Good luck. Even when you get to locate inventory you are fighting uphill with the myriad of short sell restrictions.
Does kabu.com allow short-selling of stock for residents in Japan? If so, what do they charge for the stock you borrow for short-selling a stock? For example a stock which has a large daily trade volume?