Forms 9.1 supports use variable for task participant of user task, the value of the variable need to match the user name(case insensitive). For sending email notification to the task participant, it will send email to the user's email when "Notify task participants when this task is ready" for the user task is checked. You can check the user's email from My User Settings page when login as this user:

From you description,  you should use the variable of the AD user account in the task participant for the approval task to make sure the task can be correctly assigned.  For email notification, please first  check whether the email for these users exist when check from the My User Settings page. If not, you can add an Email Service Task as parallel for the Approval Task and set Send to use the Email Address variable which is populated by the lookup to seed notification.