I haven't read the whole thread, but do we know who doctored the e-mails?
Germany have a press ombudsman, like the UK,
https://www.presserat.de/en.html. If Der Spiegel have falsified or used falsified e-mails then it must be worth making a complaint(s) to presserat about their standards of journalism.
Who can complain?
Anyone can complain to the German Press Council – whether personally affected or not, whether private individual, association or party. We accepts complaints on all kinds of journalistic work that has been published in a newspaper, a magazine or on the internet. However the Press Council does not deal with free sheets or private homepages. Also the Press Council is not dealing with radio, television or advertisements.
The complaint is free of charge. The Press Council is not dealing with legal issues.