A: Yes. If the key is valid and the game’s backend subs are still active, it works. Example: Deadpool (delisted but keys still activate).
A: Gift is direct transfer of a license; keys are external codes. Gifts can be refunded; keys cannot (by Steam). steam key activation
A: No. Many have DRM-free, Origin, Uplay, Epic, or GOG keys. Always check box label. A: Yes