There are a few things you need to reconsider in this process:
It is not possible to modify the file during the Approval process since every modification would trigger a new Approval logically. The flow needs to be stopped while you make the second change.
The ability to reassign the approval task is only available in the management dashboard. See a ...
Is the user an external user?
If it is true, it's by default SharePoint Designer workflows creating using SharePoint 2013 Workflows will not sent emails to the external users (email addresses).
If it is false, ...