Setting scope to true allows _direct_, _followers only, __unlisted_, _public_ messages be sent from Pleroma-FE. It also allows you to use subjects in your posts. By default this is set to false, which means people can't do it from the Pleroma-FE. Personally I set it to true (but left it as an option in the package). The reason why it's set to false by default is to be compatible with the GNU-social backend, but that's not an issue in this case, so I think having scopes on is better.
The cache in the nginx is something I did with trial-and-error (and eventually trial-and-succes :p ). You need to add a line in the config of the app, but also in the config of the domain. In my package that's https://gitlab.com/Spctrl/pleroma_ynh/blob/master/conf/nginx.conf