Ok, manually configuring and addition of #exec will lead to trouble. Either chose manual configuration, or stick to the FOP2 Manager (#exec).
My suggestion for you is to use the FOP2 Manager, that means that you should ditch all those user=xxx buttonfile=xx and plugin=xxx lines in fop2.cfg and leave only the #exec autoconfig-users.sh line.
Then rename your buttons.cfg file as buttons_custom.cfg
Finally log into the FOP2 Manager and create your users there, and install plugins there, via the FOP2 Manager. You will be able to install/uninstall plugins, create users and groups, assign plugins to users or not, etc, all from the FOP2 Manager web gui. And your buttons will be shown also there as you defined in the buttons_custom.cfg file.
Best regards,