@moso Thanks for your questions. :)
1. No. This i no difference than the MLAT for Switzerland. In criminal investigations, authorities work together, but German authorities have very limited permissions under what circumstances they can request what data. You'll find details here. https://tutanota.com/blog/posts/data-protection-germany/
2. No, we explain here why we do not use PGP: https://tutanota.com/blog/posts/innovative-encryption/
3. You can add an unlimited number of domains.