Hi,
sorry for the late response.
1. This can happen for some reasons. I should see the request effective error in the network tab of the developer tools to see what happendd. WMS layers works instead?
2. The file is named geostore-datasource-ovr.properties and it is located in /WEB-INF/classes folder mapstore.war archive. We usually suggest to externalize this file by following the guide
here, so you can edit the file outside the application and re-deploy future MapStore releases by overwriting the war file, keeping the configuration untouched. (
Note: the database is used by mapstore to store users, roles, maps... GeoSpatial data in general is usually provided by a separated GeoServer).
3. We usually delegate this to GeoServer. MapStore is more a client, that display data from WMS, WFS or other sources and allows to group them in map or create contents based on that data. In particular to display labels I suggest the WMS format, editing the style on GeoServer.