After paying $75, can you have the 8,500 miles back? If you can, then it is less than 1cpm, and I would probably do it.

I am not sure opening a new account for last Summer's boarding passes will work. According to this, you would need to be a member at the time of travel to get the credit. Although you were a member and had an inactive account, your to-be-open account did not exist when you traveled, and I am not sure if that would qualify you for the miles. Others will certainly know better than me.