Without getting too much into internals which could change at any time, yes. You have to replicate, partition, and serve consensus over data to achieve high-durability and availability.

For concurrent updates, the standard practice for remote file systems is to use file locking to coordinate concurrent writes. Otherwise, NFS doesn't have any guarantees about WRITE operation ordering. If you're talking about concurrent writes which occur from NFS and S3 simultaneously, this leads to undefined behavior. We think that this is okay if we do a good job at detecting and alerting the user if this occurs because we don't think that there are applications currently written to do this kind of simultaneous data editing (because Regatta didn't exist yet).