Partly because the German currency is severely undervalued for its economy (because it's held down by the other eurozone countries), favoring exports. In a sense, the other eurozone countries are subsidizing German exports. If the euro went away, and Germany switched back to the Mark, exports would be corrected downwards, and jobs would leave since Made in Germany wouldn't be profitable anymore.
Because Germany has protectionist tariffs similar to those Trump wants to implement. E.g it is not cheaper to outsource plants to Mexico and import BMW back to Germany.
Eastern European states fill that role in the EU, though I'm not sure how great the scale of that is.
Exactly - the scale is different. And where they can, jobs have already migrated from Germany