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.

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.

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.