1. Yes, as long as old blobs and old inodes are still there, changes can be rolled back, and conflicts can be merged gracefully.
2. Yes, storage will be big, and it will be larger if we don't do erasure coding. The original idea was for super important small files, like wallet descriptors or password managers, not for your entire picture gallery.
3. Yes and that's tricky, with this design outside observers can't tell that two blobs belong to the same user. 402 fixes this.