Common Data Service Connector is not leveraging CDS capabilities for alternate keys. Apart from GUID, Get Record action should accept alternate keys as well. It's a very convenient and clean way of pulling a record from CDS.
Why do we always find these things out the hard way!
It's really messed my plans up.
Great idea we need more votes!
In the meantime I think I'll build an intermediate entity which I can refer to and retrieve the contact ID
Alternate keys avoid the need to list records, loop through the result (singular) to get the GUID to then update the record. This lack of functionality, together with the inability to upsert using the CDS connector forced me to build a custom HTTP request to Dynamics web API