Hi @dennisedson and @PGVS ,
It is (currently) not possible to define what properties should be returned in the engagement API.
In your second question, you are sharing the CRM Timeline API. Based on the version in the URL, you are looking at the legacy endpoint. I can not find the v2 version in the docs.
This is an Integration endpoint that requires a HubSpot Developer app to work. You can use both oAuth or an API key.