1. web-client-config-custom.xml is for the Alfresco web client, I'm guessing you are using Share
2. There is no link between a document type and an aspect, thats the point aspects are cross type so, for example the geoloaction aspect can be added to any document type and it will then allow you to enter a Lat / Long in the metadata
3. You override it in the .properties file. This is more comlex to explain but basically make your own .properties, include it using custom-slingshot-application-context.xml and then add the same message key to this custom file as the one you want to override, restart Alfresco and bingo, new label.