Is there any mandate that Holy Mass must be offered, according to the Rite of Paul VI (Novus Ordo), with the priest versus populum for the entire duration of the Mass? Is it licit for the priest to face the same way as the congregation during the Eucharistic Prayer?
The priest may face versus deum or versus populum at his own preference. There is not any rubrics that insists he face one way or the other. There are certain parts of the Mass where the priest is supposed to face the congregation, such as during the Gospel, during various responsorials, and at the showing of the Precious Host. A complete listing can be found in the Missal. It is interesting to note that both the Rite of Mass as issued in 1970 and the current General Instruction of the Roman Missal, chp. IV part Ia assume that the priest is facing versus deum for most of the Mass because they list specific times that the priest is supposed to turn and face the congregation.
On September 25, 2000 the Congregation for Divine Worship and the Discipline of the Sacraments reafirmed that the priest may decide which direction to face to celebrate Mass. (On the Orientation of the Priest at Mass)
While not having any bearing on the rubrical correctness of orientation, it is interesting to note that the Holy Father says Mass versus deum in his private chapel.