In Mass today the priest remained seated while a layman distributed Communion. Is this allowed?
This is allowed only when necessary. Redemptionis Sacramentum explains:
The extraordinary minister of Holy Communion may administer Communion only when the priest and deacon are lacking, when the priest is prevented by weakness or advanced age or some other genuine reason, or when the number of faithful coming to Communion is so great that the very celebration of Mass would be unduly prolonged. (RS 158)
In the absence of true necessity, this practice is expressly prohibited (RS 157).