Question 1:
#28 of the most recent publication of this ritual omits “Lord have mercy, Christ have mercy, Lord have mercy.” I believe it is presumed that they should follow as the other formulations allowed match the Missal, I would imagine the intention was for this one to match as well. Should it be included or omitted?
Question 2:
Assuming that the service is occurring in the church because a priest is sick or unavailable, if the missal indicates that the Gloria and/or Creed should be said, should it also be said in the communion service?
Question 3:
If the service is taking place on a Sunday because the priest could not arrive, is there anything else that needs to be done that is different than the ritual given for daily Mass for example, use of the specific prayers of the day such as the Collect or prayer after Communion?
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1: I confess I hadn’t noticed this and neglected to comment on it in my book Eucharistic Reservation. One opinion may be that the Kyrie is presumed, but another possibility is that it is to be omitted because it is proper to Mass. Even the Missal makes a distinction between the penitential act and the Kyrie at the start of the Palm Sunday liturgy (no. 11). I lean toward omitting the Kyrie because it is not explicitly included.
2: This is easier to answer: No. This is not Mass, and those elements of the Mass do not carry into this celebration.
3: On a Sunday, under these circumstances, you use a different book: Sunday Celebrations in the Absence of a Priest. Be sure to use the 2012 edition, not the earlier one. No. 18 assigns the bishop the responsibility for working out how these celebrations are to take place. Consult his office for the details.