Thus, serving as a deacon or priest or bishop would be fulfilling an “ecclesiastical office.”
Actually episcopacy, presbyterate and diaconate are orders, and yes, with ordination comes an ontological change. Even if a priest or deacon is laicized, the ontological status does not change. Being a pastor or a chaplain or a Vicar General or a defender of the bond are examples of ecclesiastical office.
In the case of a latae sententiae excommunication, there is no requirement for formal trial or announcement; in fact, the individual brings the punishment upon himself.
True and nuanced. Sometimes for the penalty to take root, there still has to be a declaration from a competent authority. Archbishop Lefevre incurred a latae sententiae excommunication by consecrating bishops. The Congregation for Bishops’ prefect, Cardinal Gantin still had to issue a type of confirming decree.
Someone who has been excommunicated no longer has the right to the sacraments of the Church, and is not considered a Catholic unless or until the excommunication is lifted.
Certainly the person may no longer be admitted to the sacraments. Generally canonists today would say the person is still a Catholic although excommunicated. However, before the 1983 code, the understanding varied, and did include the sense of expulsion from the Church. It’s a finely nuanced point.
The question of excommunication arose earlier today at
forums.catholic-questions.org/showthread.php?t=66972, and I tried to give some background.
If he was a priest and “left ministry”, would he not then be free to marry?
Not unless the Apostolic See dispensed him from the obligation of celibacy.
but can a priest not leave the priesthood and then be entitled to the same sacraments available to other Catholics?
He also attempted marriage, and in fact, a marriage outside the Church that would have been invalid on its own. The consequences of that have been pointed out.
If he is acting as a lay reader currently, how is that prohibited to him? Because he used to be a priest?
He is still a priest, and a suspended priest at that. Suspension is understood in the canonical tradition to prevent serving in that and similar roles.