Specifically the Euro (rather than the EU) stops the German currency from appreciating massively and making their exports more expensive. That has been a massive boost to the German economy over the years.

Not only has it been a massive boost to the German economy, it has been at the explicit expense of the economies of Greece, Italy, Spain, etc. And how people can see that and be ok with it, I don't understand.

The EU under the Euro is the Fourth Reich. Nothing more, nothing less.

They could devalue their currency if they wanted to, if they had their own currency instead of the Euro

Which is what Greece, Italy etc would be doing if they weren't in the Euro. Germany wouldn't devalue as that involves increasing money supply which would drive inflation, the German post war banking system is built to have a massive fear of inflation.

True, but... other countries around the world are producing the very same things, even cheaper, but they are taxed in the EU. China has no major deal with the EU for this reason. UK leaving would open to that possibility that Switzerland decided to open in 2010, balancing what comes in and what goes out and both countries got a major boost in respective goods. Swiss got access to the manufacturing in China for Swiss high tech companies and China got access to the major luxury good produced here, plus bank services, med techs and collaborations with universities like ETHZ.

Not really. Germany has a large surplus because of the Euro. The common currency means Germany's currency is weaker than it would be otherwise thus it's exports more attractive. And countries like Greece and Spain have strong currencies than they otherwise would have. Thus their industries and workers compete at a disadvantage.

I agree that the eurozone economies are very different, and I can't see how that will be resolved without dismantling the Euro or kicking some of the weaker economies out

The US has a real central bank, automatic stabilizers, and large federal transfer payments to compensate economically weaker states for having to trade in dollars.

This is partly true but it is also a question of price. German does not pay her engineers and can just undercut competitors. Also, what do they get in exchange? Lehman certificates and Greek government bonds.

Germany has always exported a lot but also imported a lot. The crazy export started just in the 90ies. It has been very unhealthy.

But they benefit massively from being in the Euro currency. Imagine how high the Deutschmark would be. The German exporting machine wouldn't be half as effective.