I do not see a publication date field on the kav object.

Is there a way via the REST api to set it?

I need to upsert multiple records of the same type/object with one REST API call. 

I see how to upsert one record (https://developer.salesforce.com/docs/atlas.en-us.api_rest.meta/api_rest/dome_upsert.htm), but not how to upsert multiple. 

With the composite resources I see how to update or insert multiple records, but not how to upsert.

Is it possible to upsert multiple records?