Are narrative Masses where the Priest or Deacon explains what is going on allowed?
Explanations by the priest are clearly allowed. From the 2002 General Introduction to the Roman Missal (GIRM) which can be accessed from
romanrite.com/girm.html :
“31. It is also up to the priest, in the exercise of his office of presiding over the gathered assembly, to offer certain explanations that are foreseen in the rite itself. Where it is indicated in the rubrics, the celebrant is permitted to adapt them somewhat in order that they respond to the understanding of those participating. However, he should always take care to keep to the sense of the text given in the Missal and to express them succinctly. The presiding priest is also to direct the word of God and to impart the final blessing. In addition, he may give the faithful a very brief introduction to the Mass of the day (after the initial Greeting and before the Act of Penitence), to the Liturgy of the Word (before the readings), and to the Eucharistic Prayer (before the Preface), though never during the Eucharistic Prayer itself; he may also make concluding comments to the entire sacred action before the dismissal.”
Explanations are specifically listed among the duties of the deacon, from GIRM n. 171:
“d. Guides the faithful by appropriate introductions and explanations, and announces the intentions of the Prayer of the Faithful;”
Another person who can make explanations is, from the GIRM:
“105 … b. The commentator, who provides the faithful, when appropriate, with brief explanations and commentaries with the purpose of introducing them to the celebration and preparing them to understand it better. The commentator’s remarks must be meticulously prepared and clear though brief. In performing this function the commentator stands in an appropriate place facing the faithful, but not at the ambo.”
But it seems there should not be explanations in “Mass with only one minister”:
“254. Mass should not be celebrated without a minister or at least one of the faithful, except for a just and reasonable cause. In this case, the greetings, the introductory or explanatory remarks (
monitiones), and the blessing at the end of Mass are omitted.”
Explanations seem to have been discussed at the Second Vatican Council. From the GIRM, n. 13, in a section by Pope Paul VI:
"13. Indeed, since the use of the vernacular in the Sacred Liturgy may certainly be considered an important means for presenting more clearly the catechesis regarding the mystery that is inherent in the celebration itself, the Second Vatican Council also ordered that certain prescriptions of the Council of Trent that had not been followed everywhere be brought to fruition, such as the homily to be given on Sundays and holy days and
the faculty to interject certain explanations during the sacred rites themselves. [footnote 18: Cf. Second Vatican Ecumenical Council, Constitution on the Sacred Liturgy, Sacrosanctum Concilium, no. 35:3.].
The footnote, from Sacrosanctum Concilium, no. 35:3 has:
“3) Instruction which is more explicitly liturgical should also be given in a variety of ways; if necessary, short directives to be spoken by the priest or proper minister should be provided within the rites themselves. But they should occur only at the more suitable moments, and be in prescribed or similar words.”