(I’ll deal with your topics one at a time.)
What was the reasoning behind moving or removing the … altars to the BVM and St. Joseph?
1794 - Pope Pius VI -
Auctorem Fidei, 31:
31.
The proposition of the synod enunciating that it is fitting, in accordance with the order of divine services and ancient custom,
that there be only one altar in each temple, and therefore, that it is pleased to restore that custom, –
rash, injurious to the very ancient pious custom nourishing and approved for these many centuries in the Church, especially in the Latin Church.
1963 - Vatican II -
Sacrosanctum Concilium, 128:
Along with the revision of the liturgical books, as laid down in Art. 25, there is to be an early revision of the canons and ecclesiastical statutes which govern the provision of material things involved in sacred worship. These laws refer especially to the worthy and well planned construction of sacred buildings,
the shape and construction of altars …]. Laws which seem less suited to the reformed liturgy are to be brought into harmony with it, or else abolished; and any which are helpful are to be retained if already in use, or introduced where they are lacking.
1964 - Consilium for the implementation of
Sac. Conc. -
Inter Oecumenici, 93:
There are to be
fewer minor altars and, where the design of the building permits, the
best place for them is in
chapels somewhat set apart from the body of the church.
2002 - General Instruction to the Roman Missal, 303:
In building new churches,
it is preferable to erect a single altar which in the gathering of the faithful will signify the one Christ and the one Eucharist of the Church.
Vatican II did not say that there should only be one altar. That said, the reason for the emphasis on a single altar – or, more properly, a distinct main altar – is that it is a sign of unity. However, as the late Bishop Fulton Sheen put it:
Picture then the High Priest Christ leaving the sacristy of heaven for the altar of Calvary. He has already put on the vestment of our human nature, the maniple of our suffering, the stole of priesthood, the chasuble of the Cross. Calvary is his cathedral;
the rock of Calvary is the altar stone; the sun turning to red is the sanctuary lamp;
Mary and John are the living side altars; the Host is His Body; the wine is His Blood. He is upright as Priest, yet He is prostrate as Victim. His Mass is about to begin. (Calvary and the Mass)
(sometimes the pastor just can’t win).
).