What is the ruling on the regular Sunnah prayers of Fajr and the pre-Dhuhr prayer for one who prays the obligatory prayer alone?

The قبليّة (pre-Fard) regular Sunnah prayer is performed before the obligatory (Fard) prayer. So, whoever wishes to pray the Fajr obligatory prayer, whether alone or in congregation, should first pray its regular Sunnah. This also applies to the Dhuhr prayer.