Dear Masood,
1. Yes, In Access Request, you would be assigning the Roles to users. It is best practice to BRM before you Implement Access Request.
2. If you are using the MSMP workflow then you are required to set the approver at the stage level of the MSMP. Approver will get the work Item in its Inbox. you can also make use of BRF.
3. You are required to maintain the connection with the HR system or LDAP to maintain the manager.
otherwise, there will be no use of manager field even If you add manually.
Thanks & Regards,
Ashish