Are the two rak'ahs of circumambulation after 'Umrah independent, or can they be a Sunnah for a congregational prayer, and what is their ruling if the circumambulation is after the 'Asr prayer?

The two rak'ahs of tawaf are a confirmed Sunnah, even if the tawaf is obligatory. They are sufficient in place of an obligatory prayer, and this is the preferred view. It is permissible to perform them during a prohibited time for prayer.