Does the grave being a garden of the Gardens of Paradise mean forgiveness of sins, or will accountability and punishment for them occur before entering Paradise?

Sinful believers will see their places in both Paradise and Hell: their place in Paradise is their final destination, and their place in Hell is their temporary abode of torment. The former is what they will ultimately reach after being purified from their sins, and the latter will vanish after the torment. This does not mean they will enjoy bliss in the grave; rather, their bliss might be limited to seeing their place in Paradise, while obedient believers will enjoy the expansion of their graves and other forms of bliss.