Japan's population is 120 million and its exports are 0.5 billion. So what? What does it give us? I don't see anything.
I'll restate again. China devaluing its currency is another issue. When you are world's largest debtor in fiat currency, that you yourself print out, than you can print as much of it as you want to devalue your debts and thus con the lenders. The price for that will be the fall of living standards for US citizens, who'll see also their savings devalued, but other than that its not a big deal.
As for your second point, I believe, reserve currencies are of secondary importance - US dollar will remain an important reserve currency, until major oil countries do oil deals in USD. If the latter ends, reserve currencies will not save the USD. That might one of the reasons, why the US is taking agressively oil trade in its own hands and trying to bully the Middle East into submission.