I would like to know which ArcPy functions can be used to read metadata from existing datasets in ArcCatalog?
The ArcPy functions for working with metadata are found in the Metadata toolset:
All ArcGIS items have a description, which is also referred to as metadata. The Metadata toolset lets you handle metadata for ArcGIS items and stand-alone metadata XML files.
Each tool in the Metadata toolset has a Code Sample to illustrate how it can be used in ArcPy.
I am not a metadata expert but I think the tool you need first is Validate Metadata which:
Exports metadata to a standard metadata format then validates the exported file.