My best guess is that the issue is related to file permissions. Currently insync-headless is running under user account “server” and “server” happens to be in group “client” with user “client”. The directory space assigned to insync is /home/“server” and the user "client"s files are being sync’d under /firstname.lastname@example.org. All the files in directory email@example.com belong to the namesake group (ie, chgrp -R client /firstname.lastname@example.org) and all new files being written to that directory automatically belong to the same group via chmod g+rwxs. Because email@example.com is actually a samba share for the very same user, all new files being created by the user actually belong to client:client, and not server:client. However, this shouldn’t be an issue because all new files create as drwxrwx–x and “server” belongs to the “client” group.
To throw a wrench into my theory though, I created a directory named test and a file named test.txt that was explicitly created by “server” under the client’s samba share. Even though user “server” is running insync and those files are owned by “server” it still only created the directory and didn’t move any files. I’m at a loss right now as to why I can’t sync files to GDrive. The only way to sync new files is for me to delete my account and readd it.