The Discogs Release Country will be on the UD Text editing panel.

If you want you access the value in an action you can use a 'User Defined Fields' statement or read the value directly via an escape sequence:

\^Discogs Release Country^ (backslash before the first ^)

You can also add it to a text field in an action via the Insert Common UDTI context menu item.

Yes. You can access any UDTI this way. Yate maps the FLAC version of the tag to DISCOGS_ALBUM_RELEASECOUNTRY and hides it as it is a normalized mapping across audio file types. However, if you have older values named RELEASECOUNTRY you can access them in the same manner.