The celebrant’s position and proximity to the participants We know that the apostles were present in a face to face relation to Jesus at the Lord's Supper. 
When did the celebrants of the later Eucharist turn their backs on the participants, and why? Also when were the elements removed from proximity with the liturgy (or mass) participants, into the separate or remote altar or sanctum, that we observe in the Orthodox and Traditional Latin Rite churches of today?
The celebrant’s position and proximity to the participants?

There is certainly not enough historical evidence to give an exact date on this usage within the Holy celebration of the Mass. It may in fact have come from the Apostles, but we just do not have the proper data at hand. 

What is certain, is the fact that this practice existed very early within the Church, along side of the priest saying Mass facing the people.


  Catholic Teaching and Liturgical Tradition
  
  First, let’s consider what the Church currently teaches about the the priest’s orientation at Mass. In September 2000, the Congregation for Divine Worship (CDW) responded to the question whether the General Instruction of the Roman Missal should be read as dictating that the priest must always say Mass versus populum (“facing the people”). In a formal document signed by the prefect and the secretary, the CDW explained that the answer was “no.” Priests can also say Mass versus apsidem (“facing the apse,” which is the front of the Church). More important, whatever physical direction the priest faces, “his spiritual attitude ought always to be versus Deum per Jesum Christum [toward God through Jesus Christ]” (cf. GIRM, 78).
  
  Second, let’s consider the teaching of the Second Vatican Council. Although the Council did not speak about the priest’s physical direction at Mass, it did say that what we do in the liturgy is a consequence of who and what we are before God. Since the human body “shares in the dignity of the ‘image of God’” (Catechism of the Catholic Church 364), “in the Liturgy the sanctification of the man is signified by signs perceptible to the senses, and is effected in a way which corresponds with each of these signs” (Sacrosanctum Concilium, 7). That action is principally “an exercise of the priestly office of Jesus Christ” before the Father in heaven (Sacrosanctum Concilium, 7), while “in the earthly liturgy we take part in a foretaste of that heavenly liturgy which is celebrated in the holy city of Jerusalem toward which we journey as pilgrims” (Sacrosanctum Concilium, 8).
  
  There are two primary ways in which Christians in their worship in the first centuries of the faith embodied this communal pilgrimage toward heaven:
  
  ‐ By facing one another for proclamation and dialogue. The priest in the patristic era faced the people during the Liturgy of the Word (as we now call it) so as to fulfill his role, which was to act in persona Christi, both when proclaiming the gospel to them and during those parts of the Mass when he prays in dialogue with them.
  
  ‐ By facing east for the eucharistic prayer. As Pope Emeritus Benedict XVI points out in his book The Spirit of the Liturgy, priests in the patristic era almost universally faced east, the direction of rising sun, during the eucharistic prayer, anticipating thereby the glory of Christ’s return to earth. In most churches, this also meant that they faced the apse, with the people. Acting again in persona Christi, priests embodied the fact that Christ the Priest became completely as one of us when he journeyed toward the cross and invited us to follow him. - Cardinal Robert Sarah proposes that the Church recover a piece of its tradition.


At some point in time, the tradition of saying Mass ad orientum became the norm. When this exactly happened is lost to history.

The tradition of facing East is as we see was well established in the Early Church, if not in the Apostolic Era itself. 

Here is what the Catholic Encyclopedia has to say on this subject:


  Orientation
  
  The custom of praying with faces turned towards the East is probably as old as Christianity. The earliest allusion to it in Christian literature is in the second book of the Apostolic Constitutions (200-250, probably) which prescribes that a church should be oblong "with its head to the East". Tertullian also speaks of churches as erected in "high and open places, and facing the light (Adv. Valent., iii). The reason for this practice, which did not originate with Christianity, was given by St. Gregory of Nyssa (De Orat. Dominic., P.G., XLIV, 1183), is that the Orient is the first home of the human race, the seat of the earthly paradise. In the Middle Ages additional reasons for orientation were given, namely, that Our Lord from the Cross looked towards the West, and from the East He shall come for the Last Judgment (Durand, Rationale, V, 2; St. Thomas, Summa Theologica II-II:84:3). The existence of the custom among pagans is referred to by Clement of Alexandria, who states that their "most ancient temples looked towards the West, that people might be taught to turn to the East when facing the images" (Stromata, vii. 17, 43). 


Cardinal Ratzinger held the tradition of facing East in high regard.


  Five years before he became Benedict XVI, Joseph Ratzinger wrote that, notwithstanding various liturgical innovations, “one thing has remained clear for the whole of Christendom: praying towards the east is a tradition that goes back to the beginning.” As he wrote in The Spirit of the Liturgy:
  
  
    The common turning toward the east was not a “celebration toward the wall;” it did not mean that the priest “had his back to the people.” . . . For just as the congregation in the synagogue looked together toward Jerusalem, so in the Christian liturgy the congregation looked together “toward the Lord.” . . . They did not close themselves into a circle; they did not gaze at one another; but as the pilgrim People of God they set off for the Oriens, for the Christ who comes to meet us. - Facing East