You are correct snapmirror on the target is read-only, in order to get read-write access you would need to clone a snapshot on the target. Same thing goes for snapvault, it will be read-only on the target and in order to get read-write you would clone the baseline snapshot.
What you could maybe do with SnapCreator is have SnapCreator take snapshot->snapmirror update->clone secondary using recently transfered snapshot. The problem is if you then want to make changes on the target (USA) and update source (Germany) it becomes rather complex and I am not sure how that could be accomplished.
I think what you are trying to achieve is complex, what would you want to happen if the same file was modified at the same time in the different locations.
If your needs are simply if you could schedule a Robocopy each end.
What you probably need is a third party DFS (distributed file system) or GNS (global namespace) product, NetApp used to sell VFM (Brocade's StorageX) for this but that has gone the way of much their client-side software and is being discontinued. Other products in this market are F5's Acopia and EMC's Rainfinity.
Just to add another product to the mix IBM's SONAS also provides global namespace with replication and read/write capabilities for a single mount replicated to multiple data centers. One thing to keep in mind is that SONAS is targeted for more high performance requirements where as the EMC Atmos offerings in this space are more low/mid performance.
Our company, Peer Software, is a NetApp Advantage Alliance partner and we make a product named PeerLink that does exactly what your project requirements require. PeerLink provides real-time bi-directional synchronization between two NetApp filers (or any combination of NetApp filers and Windows Servers) so that you have Read and Write Access on BOTH filers. Most of our customers do this so that shared project files are localized at each office location filer and file access speeds are at LAN speed quality. Version conflicts are prevented through file locking, which means that when a file is open for WRITE access on one filer, our software will then lock down the file on the other filer for READ ONLY access.