I’m not a reader of eBooks, but Voi provided a comparison of BookWalker and eBook Japan that came down heavily in favour of BookWalker here (including the existence of a Google Play app in the US):
Kobo has a Google Play app, and @ChristopherFritz uses it so may be able to elaborate.
I don’t know anything about BookLive, I’m afraid.
For what it’s worth, I believe that if you do use a VPN for Amazon JP for just a couple of purchases they’ll leave you alone again.