Elasticsearch 6.7.0 Homebrew install macOS 10.14.4 fails to run ERROR: Cluster name [elasticsearch_nathan] subdirectory exists in data paths

The error message you shared in Unable to start Elasticsearch - java.lang.IllegalStateException: Duplicate key org.elasticsearch.plugins.PluginsService$Bundle@e3623fe is different.

Which one is the most accurate now?

Here the message seems to indicate that you upgraded from an old version, did you?
If you don't have important old data and are ok to drop it, you can just delete the elasticsearch_nathan dir.