No, this is not a zero sum game. Germany just makes better stuff. It's not cheating. If everybody else did the same, the world as a whole would be wealthier and the imbalances you're talking about would smooth out.
The motivation of trade is simply comparative advantage. Trade is beneficial regardless of whether the products being traded have equal value.
If other nations became more productive, the German current account surplus would shrink and the global productivity level, by implication, would be higher than now. My argument rests on a simple assumption: Higher productivity equals greater wealth. This can of course be questioned on a philosophical level, but not on the basis of mainstream economics.
I don't, but I believe someone has to buy this production or increased productivity doesn't help anyone.
Your assumption sounds correct, but isn't the base of Germany's economic growth in the last years: Germany didn't increase productivity (at least not by a significant margin), but decreased wages (compared with it's neighbors). And this decreased wealth.
I don't deny at all that there are many short term effects and flucuations that have nothing to do with productivity. But we're talking about the different speeds of European economies and those different speeds correlate very well with different levels of productivity.
And look at the Greek stock market. The biggest non financial companies there are a Coke bottling company and the lottery. The biggest German listed company (I believe) is Siemens, which makes high tech industrial equipment like power plants or health care equipment. The Chinese want that. They need energy, they don't want to play in the Greek lottery and they can bottle their own Coke. That's the reason for the two speed economy.
I agree with your argument -- IIUC you're invoking comparative advantage. But the two concepts above are fundamentally different. Wealth is a stock, while productivity is a flow multiplier (e.g. A(t) in Solow model: http://en.wikipedia.org/wiki/Solow_residual). Saying "equals" is a bit adventurous, IMHO. Probably a bit pedantic, but then you do stress that that statement is on the basis of "mainstream economics".