Coal Hammer on Stone -> Extra Drop
Coal Hammer on Dirt -> only dirt drop

Is the morevanillalib-server configuration file supposed to be in the "config" or "defaultconfigs"/"serverconfig" folder?

it should be in the serverconfig. I just put it in the normal config path for easily accessing it and modify.

Maybe that's why it's not working. I changed the configs in the "config" folder and created a singleplayer world. Maybe the changes get overruled by the file in the "world"->"serverconfig" folder which is created on singleplayer world creation.
Maybe the config has to be in the "defaultconfig" folder to be accepted and not getting overruled by the file in the "serverconfig" folder.

-> #8 :)