Here is the actual decree. Note there is NO requirement to "confess at the Cathedral" in order to obtain the plenary indulgence, only the obligation to "visit the Cathedral of Agaña, under the title of the Most Holy Name of the Blessed Virgin Mary, in the form of a pilgrimage and there devotedly participate in the jubilee celebrations..."
Decree from the Holy Father Granting Plenary Indulgences
The Apostolic Penitentiary, in virtue of the faculties granted to it by Our Holy Father Francis, grants a Jubilee Year with the connected plenary indulgence, with the customary conditions (sacramental confession, eucharist communion, and prayer for the intentions of the Supreme Pontiff) rightly fulfilled, to be gained by the truly penitent faithful, which may be applied even to the souls in Purgatory by way of intercession, if they should visit the Cathedral of Agaña, under the title of the Most Holy Name of the Blessed Virgin Mary, in the form of a pilgrimage and there devotedly participate in the jubilee celebrations, rightly determined by His Excellency, the Most
Reverend Metropolitan Archbishop, or at least through a suitable space of time will have been occupied with pious considerations, including the Lord’s Prayer, the Creed, and with invocations of the Holy Virgin, the Mother of God. Those pious faithful who are prevented by age or grave sickness, may equally be able to attain the plenary indulgence, having arrived at a detestation of whatever sin, and having formed the intention, where it should be possible, of fulfilling the three conditions, if they should add themselves spiritually to the jubilee celebration and the pilgrimages, having offered their prayers and sufferings to a merciful God.
So that access to the divine gift through the keys of the Church might be rendered easier by pastoral charity, this Penitentiary earnestly asks that priests endowed with suitable faculties for hearing confessions, might offer themselves in the cathedral church with a prompt and generous spirit for the celebration of Penance and that they might often bring communion to the sick.
The present [stipulations] are to be in force throughout the Golden Jubilee of Agaña, everything to the contrary not in the least withstanding.
Given at Rome, in the halls of the Apostolic Penitentiary, on the 22nd of September, A. D. 2014. Maurus Piacenza, Cardinal of the Holy Roman Church Christopher Nykiel, Regent Decree from the Holy Father Granting Plenary Indulgences.
"It is appropriate, but not necessary, that the sacramental Confession and especially Holy Communion and the prayer for the Pope's intentions take place on the same day that the indulgenced work is performed; but it is sufficient that these sacred rites and prayers be carried out within several days (about 20) before or after the indulgenced act."
http://www.vatican.va/roman_curia/tribunals/apost_penit/documents/rc_trib_appen_pro_20000129_indulgence_en.htmlAlso see my post: THE AGANA INDULGENCE. LUTHER REVISITED?