No, that's not how it works. The OS must be restored back to the device before the backup is restored to it, and an OS that isn't getting signed won't go on the device.Save your phone backups to Time Machine, and you can revert back at any time to 7.x, should the need arise, without having to worry about digital signing.
I believe you can use the reset settings and erase content option on the phone.My iPad Mini Retina currently has iOS 7.1.2 (I never upgraded to 8.x). I would like to restore it and keep the iOS at 7.1.2 -- I'm selling the iPad Mini Retina, and a potential buyer doesn't want iOS 8.x. Since the firmware is no longer being signed, does this mean that I'm screwed?