One way to do this would be to have a hidden, read-only field on your form called Approver and set its default to Current User. That way, when the form is loaded by someone, their user name will be populated into that field and you can use that field later for routing decisions.

You could even have two hidden fields, one for Approver, then another for Role that could be populated by a lookup. Keep a database of users and their roles, and search for the username to populate the Role field. That way, if you add other people into this process, you just have to give them a role in the database and the process will continue to work smoothly.