SuscipeMeDomine, if what you say is true, then all those people who come after the priest begins his procession down the main aisle, or anyone who leaves before the priest exits, has NOT fulfilled his/her obligation, correct?
CCC #2180 states:
The precept of the Chuirch specifies the law of the Lord more precisely: “On Sundays and other holy days of obligation the faithful are bound to participate in the Mass.” “The precept of participating in the Mass is satisfied by assistance at a Mass which is celebrated anywhere in a Catholic rite either on the holy day or on the evening of the precdeding day.”
That sets up #2181:
The Sunday Eucharist is the foundation and confirmation of all Christian practice. For this reason the faithful are obliged to participate in the Eucharist on days of obligation, unless excused for a serious reason (for example, illness, the care of infants) or dispensed by their own pastor. Those who deliberately fail in this obligation commit a grave sin.
So, to my reading, one is obliged to participate in the Liturgy of the Eucharist, but need not necessarily participate in the Liturgy of the Word (though of course,this is necessarily very desirable and warmly recommended.).