1. Is it true that SFTP only encrypts the command channel but not the data channel?
No. There's no command/data channel as with FTP, there is only one channel and it is whole encrypted. The above may be true for FTPS (FTP over SSL), but I do not know.
2. Do you have any plans on adding rsync or unison file synchronization capability that would handle the synchronization process?
3. Does SFTP only compare timestamps to determine if a file has changed
Yes. There is no other way with SFTP. The latest version 5 of SFTP specification defines MD5 hashing function, but AFAIK there's no server that supports it yet.
and if so, does it copy or syncs the newest version of the file in its entireity? (No underlying diff/patch algorithm)
The above is not supportted by SFTP.