Maybe I'm misunderstanding, but if the document in Google Drive is served over HTTPS, then the referrer when a user visits a linked site should only show the hostname (ie drive.google.com) not the full URL, right?
Sounds like you're actually looking for something pretty complex if it needs all of those features. We use Asana for managing remote teams and that's worked well for us. If it needs to be something that lives inside GMail, maybe check out Streak. It's a lightweight CRM that let's you manage different processes, flows and relationships from within your inbox.