i have to approve a custom obj record which has it's status field to completed.
Now a process builder is setup which will trigger the approval process setup..
this approval process will send an email notification and upon approval by a queue called 'service group' this status field will update to approved.
this email notification has an email alert which has a case to email id in the additional email addresses. now a new case is created and this email is attached to that case.

Now the requirement is: 
-service group queue shall approve the record from the created case. because the service group does not have access to the custom obj.
-the way they need to approve is by replaying as APPROVED.
any queue user can do this.

challanges i'm facing are:
-i cannot use the queue in the approval process because there is a master-detail relationship and this custom obj is on detail side.
-how can i make sure that APPROVE replay from the case attached Email will tick this approval process record. i dont see the auto approval Email id (the salesforce automated one) in the To address of the Case Email.
-since i cannot use the queue and anyone from the queue can approve these records how can i choose the approver in the approval steps.

-What is custom Email Template for auto email approval and how can i use it here?

The question is on the lengthy side. but please help me if you can.

Thanks in advance.