7.5lb at 9 months is a little on the light side depending on her body habitus, but I've had adult girls who only weighed in at 7lbs. I can't comment on diet recommendations since cats have different dietary needs than humans so wouldn't know what would be appropriate. Possibly your vet can help you there?
As for the testing - no one can make that decision for you. If it was me I would want to know right away what I am dealing with, but I understand that you and she have both been through a lot and probably need a mental vacation from all the stress.
As for the testing - no one can make that decision for you. If it was me I would want to know right away what I am dealing with, but I understand that you and she have both been through a lot and probably need a mental vacation from all the stress.