You have to make sure you installed manjaro-settings-samba and rebooted your system, then all that will work. Also mentioned in the wiki
Probably you want to use something else instead of WORKGROUP, so make sure you check that in /etc/samba/smb.conf after you install the mentioned package.
That is a loaded question, not going to answer it.
There are other sharing solutions that many users prefer over samba. Example: