1. Yes.
2. tftp is the username that the server runs as and not used to actually login. tftp does not have any security or authentication.
3. The configuration file is written in English and stored as an ASCII file. You can change the TFTP_DIRECTORY location to anything but it will have to have the same permissions.
The TFTP_USERNAME should be left as is.
TFTP_ADDRESS=":69" basically means it can be access by all network connections on port 69 and should be left as is.
https://linux.die.net/man/8/tftpd