My argument is that it was the wall of seperation between jew and gentile (which paul fought to break down) that occured during that time in the infant church when more people of the gentiles were coming into the church (in leadership roles) that the apostasy happened.
You mentioned the breaking down of the wall of separation between Jews and gentile as easy entry zone of a supposedly happened "apostasy".
However right as regards to the Sabbath:
"...Jews in the first century understood that God did not require Gentiles to keep the Sabbath. When the barriers between Jews and Gentiles were eliminated through Jesus’ death (Ephesians 2:13-16), the Sabbath was one of the barrier ordinances eliminated. It was a ritual law, not a timeless and eternal moral law. ...
...Paul tells us that the gentiles, even without the written law, had a law written on their hearts (Romans 2:14-15). They could know by nature that honesty was proper. But in contrast, they could not know from nature that they should anoint the right thumb instead of the elbow. They could not know by nature that they were to avoid work every seventh day.
Quotes from: "Is the Sabbath a moral law or a ceremonial Law?"
Thus just Paul himself and just the breaking down of the barrier made it necessary not to load jewish burdens on gentiles. Likewise the Apostolic Council didnt mention the Sabbath as duty for Gentiles and Gentile christians.
According to the source below it was about 130 A.D. that jewish christians stopped celebration the Sabbath. But this didnot happen in this epoche as a replacement of Sunday to have a new day of rest, but it simple went out of practice. Christians worked on this day in the second century.
Maxwell: Many Christians were already honoring Sunday near the beginning of the second century…. Evidence is very strong…that many if not most Christians had given up the Sabbath as early as A.D. 130…. Just as Sunday observance came into practice by early in the second century, so among Gentile Christians Sabbath observance went out of practice by early in the second century.
But this was not a replacement for the Sabbath:
Sunday was observed only as a day for worship, not as a Sabbath on which to refrain from work…. Sunday was not at first celebrated as a ‘Sabbath.’… It was not observed in obedience to the fourth commandment…. Sunday was regarded by Christians generally not as a day of rest or holiness but as a day of joy.18
Sabbath and sunday in the Early Second Century
From a german source (googletranslation): https://bibelbund.de/2015/07/von-der-heiligung-des-sonntags/#fn10-3957
At first the Christians, as far as they came from Judaism, still held the Sabbath. At least until the destruction of Jerusalem by the Romans it can be seen that the Jewish Christians in Jerusalem and Israel have consistently adhered to the Jewish ceremonial law. One of the reasons for this assumption is that "we do not hear of any persecution from the beginning, because Christians did not follow the Sabbath."
James, the brother of Jesus, who had stood at the head of the congregation in Jerusalem until the destruction of Jerusalem, was given the epithet "the righteous" by the Jews of the following century, not least because of his legal severity and his zealous temple service. The observance of the Sabbath for the figure of Jewish life at that time was too strong and meaningful, as "that the oldest Christianity, in the same way, had distinguished the Jewish week in this way, as opposed to its other day. The Sabbath was a strong bond of communion, which linked it to the whole people, and by keeping the Sabbaths holy by the Jewish Christians of Palestine, they followed the example and instruction of Jesus, who was not in the role of a rebel, but of a sensible accomplisher Legal orders ".12
But at the same time, the celebration of the first day of the week as the 'Lord's Day' is formed. On the "Resurrection Day" they held their own meetings. The celebration of the 'Lord's Supper' was at the center. Since the resurrection took place on the day after the Sabbath (Mark 16: 1ff.), The Lord's Supper was also celebrated on this day.13 From the celebration of the Lord's Supper, the first day of the week may have been given its early Christian designation "Day of the Lord" (Rev. 1.10).
It is of decisive importance for the origin of Sunday that among the four points, which were ordinarily ordered on the so-called Apostolic Council, Apg 15, for the Gentile Christians, the salvation of the Sabbath is no more the same as the foundation of another holy day ( Acts 15:28). Paul is vehemently opposed to any attempt to persuade the Gentile-Christian communities to hold certain days (Gal 4: 8, Col. 2: 8ff.), For they represent for him a relapse into a life of works (cf Gal. .
Under the decisive influence of Paul crystallized in the communities of Kleinasiens, Greece and Macedonia clear traces of a worship celebration. Thus Paul proposes to the church in Corinth what has already proved its worth in the communities of Asia Minor to complete their promised collection of money for the impoverished congregation in Jerusalem on every first day of the week when they are already together (1Cor 16: 1f.). In Troas (Acts 20: 7ff.) We find the celebration of the Lord's Supper together with a preaching service. Further concrete indications of the form and content of worship meetings can be found in the introductory wording in the first corinthian letter: "... if you come together ..." (cf 1 Cor 11: 20ff, 14, 26ff.
Nowhere did the early church call the Sunday 'Christian Sabbath' or argue its celebration derived from the Sabbath.
When the Apostle John moved to Kleinasien around the year 70, he found the celebration of the Lord's Day there as given. Thus, in revelation, he speaks of it quite naturally (cf. Rev 1: 10, "I was on the Lord's day in the Spirit ..."). Nowhere did the early church call the Sunday 'Christian Sabbath' or write that its celebration derived from the Sabbath.
"All of the testimonies indicate that the separation from the Sabbath already began in the Jewish Christian community, albeit initially in a twist that the heathen Christian communities were never committed to the Sabbath (Acts 15) and that the celebration of the Lordsday was finally settled under occasional conflicts (Gal 4: 8ff, Col 2: 8ff.) And relapses (Letter of Ignatius to the Magnesians 9:13). "
I must say what regards me, i myself am researching this matter from time to time, but I always give up because i am confused.
I read much about Sabbath of Adventist, but i dont like the thought that a sevenday-rythm is a moral or universal principle.
this german information from a "voice of hope" obviouls adventistic, is called "the Sabbath Conspiracy" http://www.bibelstudien-institut.de/fileadmin/bibelstudien-institut.de/downloads/minibuecher/Minibuch_Nr_4_Sabbat_Verschwoerung.pdf