I have a library about 5 levels down from the site collection top level that has three groups with access. I will probably be modifying those groups' permissions, adding more groups, etc. over the next couple of weeks, in a fast moving legal environment with many stakeholders But I need 5 external users to be able to see just one of the documents in that library. Can I send the link to just that doc, set up somehow just for them to see it - without breaking inheritance? (with a new group is fine) I can't do code, just regular site admin steps - you were a great help last week - simple does it!
No, not without breaking inheritance.
If you don't break inheritance, then you can't assign permissions to your document without also assigning permissions for whatever parent is being inherited from.
If however you're able to break inheritance, then you can just use the SharePoint 'share' option (SP2013, anyway) which will break inheritance, copy all the permissions, and add the new user with Edit permission.