If it becomes necessary for a celebration to be carried out in the absence of a priest, then no one person should "officiate" or "preside." The recently released instruction Redemptionis Sacramentum states:
"It is necessary to avoid any sort of confusion between this type of gathering and the celebration of the Eucharist. . . . It will be preferable, moreover, when both a priest and a deacon are absent, that the various parts be distributed among several faithful rather than having a single lay member of the faithful direct the whole celebration alone. Nor is it ever appropriate to refer to any member of the lay faithful as ‘presiding’ over the celebration" (RS 165).
Regarding preaching at these celebrations, only the bishop may give permission for a layperson to preach—and only under extraordinary circumstances, such as the absence of sacred ministers.
"The homily on account of its importance and its nature is reserved to the priest or deacon during Mass. As regards other forms of preaching, if necessity demands it in particular circumstances, or if usefulness suggests it in special cases, lay members of Christ’s faithful may be allowed to preach in a church or in an oratory outside Mass in accordance with the norm of law. . . . All must remember besides that the faculty for giving such permission belongs to the local ordinary, and this as regards individual instances; this permission is not the competence of anyone else, even if they are priests or deacons" (RS 161).