Background: We have a custom Opportunity Object with a related Content List (documents like Solution Plans pertaining to an opportunity is uploaded into the Content via the related list). Few of the documents uploaded should go through a review before it is finalized.
We are trying to develop an Approval Process with the below characteristics:
 Choose an Approver - Salesforce User and/or non Salesforce User  Choose a document from the Content and send it as an Attachment via the Email (alternatively the document link will also work)  Capture the comments via Email (Email response)
After investigating we found the following limitations:
 Non Salesforce Approvers cannot be chosen  Approvals cannot have attachments  Email response is only available for Salesforce Users (need to have "API Access" in profile)
To develop Apex controller(s) to handle attachments via Email to salesforce & non Salesforce users;
Is there a way to achieve the below:
 Capture Review Comments (from both Salesforce & Non Salesforce Users) as we are not using the OOB Approval Flow.  Send document link uploaded in Content rather as an Attachment.