The error messages is limitation of the current development. There was not mentioned any specific about plugin's name (start or end part), so all files in the plugin's folder considered by application as plugins, obviously that not plugins (or part of other plugins) cannot be loaded into OBS Studio. Application informs user about this via the log error and warning messages during application startup.
As for the Preview and Program labels - all this is hard to implement in current version of the Multiview. You may wait for years to see the improved feature.
What scenes, from the mentioned list of scenes (from the log-file), is not labeled in your Multiview? Is there a scene numbers, at least?