I think it's still complicated. I wouldn't try to play too much with it, but you can use the "same" account across each of their national portals except parts of it seem to be local to the country, and many amazon apps don't have a notion of country (e.g. when logging to the prime app you only use the email and password, and it guesses from there. It actually finds the right item from an app downloaded from a different country's app store).

I made a point to separate mail addresses by country to avoid getting hosed, but I'd imagine the fun trying to access Prime or kindle purchases from an account that has them in multiple national stores.