When
englishal
No it dosn't gey backdated. If your validation expires and therefore you do a Renewal Test then the next validity period starts from the date of the successful Renewal Test.
Yes, you can be tested on a group share aeroplane that is on a private category. You can't train on it though. Whether you can do the dual training 'Revalidation Flight' on a group aeroplane is unclear and open to some interpretation it seems.