Here are some utility functions I have put together which might be useful in the future.
I have used the SharePoint Online CSOM version 16.1.5626.1200 for this:
Also, there are some really nice samples in SharePoint Patterns and Practices (PnP) around External Sharing
1) Site Collection External Sharing
2) External Sharing Expiration Service
In my case however, I have used the native CSOM API methods for creating the utility functions.
1) Get Externally Shared Documents
This method uses search to get all documents which are externally shared. You can modify the search query to filter by site, site collection, document library etc.
2) Get external users for a document
This method uses the native CSOM API to get all the users with whom a particular document is shared.
3) Get all external users in a Site Collection:
4) Get all external users in a Tenant:
Hope this helps!