The wall of separation wasn't the law. The wall of separation that paul sought to remove was religiously imposed customs that said that jews and gentiles shouldn't "mix company". He even called peter out for playing the hypocrite because he returned to his former way. On learning the truth that God does not call gentiles "unclean" (sheet incident) he started having contact and ministering to them, eating etc. then when "certain men from james' company" came to jerusalem he separated himself again in the "manner of the jews"...even teaching this separation, according to the text. That's what was separating the Body...paul taught that there was no longer jew or gentile, free or slave because all are one in Christ. Much like in OT times when gentiles joined the jews they were required to uphold the law, they became one with israel...israelites.
The burden the fathers couldn't bear wasn't the law either. The penalty for breaking the law, however, is a different story. Moses murdered a man...without the grace of God in the sacrificial system God's justice would have seen him forfeiting his life for his crime. The NT says that the law is holy, just and perfect. scripture also says that God doesn't put more on man than he can bear. It would be wholly unrighteous of God to require obedience to a law that he knows they couldn't "collectively" ever keep. Jesus rebukes the pharisees for placing burdens on the people by way of contravening the law. Jesus says that He didn't come to abolish the law but to "fulfill" it. And that anyone that teaches it is done away with will be least in the kingdom.
Paul said that he wouldn't have known what sin was if it wasn't for the law and in 1 john 3:4-5 scripture says that sin is lawlessness...or trespassing of the law. 1 john 2:1-2 says that if we do sin or trespass the law, Jesus is our advocate with the Father. Jesus Himself showed this with the woman caught in adultery. He advocated for the woman and then told her to sin no more...as in stop trespassing the law.
Remember scripture says (to paraphrase) that the law is only a problem for people who don't obey it? That's because being "under" the law means you will be subjected to the penalties/requirements of the law. If you uphold the law you are not "under" it...you don't feel its weight, so to speak. Jesus took away the list of penalties/requirements which were against us as sinners who trespassed against the law. His yoke is light because we are no longer held to the penalties of the law...that weight that so easily ensnares us. (We are justified by our faith in Him accomplishing that. Abraham was justified by his faith that God would provide a lamb...he believed God's promises to him and so knew that his son would be spared) Jesus paid for those sins in full, having nailed them to the cross when He became sin for us. He satisfied the penalty for sin. He didn't abolish the law. Galatians 3:10-14 says the MAN who transgresses the law is cursed...it doesn't say the law is a curse. Again, Jesus became the sinner on the cross, He didn't become the law on the cross.
--"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 "--
By his own admission, paul didn't teach anything contrary to the law. So he couldn't have taught it was replaced by grace for the gentile. For one thing the teaching that is opposite the law is lawlessness, not grace. Galatians 4:8-11. Paul here is saying that before they knew God they served OTHER gods (which are weak and beggarly) by way of observing days, months, and seasons...he fears that they want to return to their bondage to those gods. This in no way says that the feasts ordained by God are weak and beggarly. To the contrary, it was during these holy convocations that God said He would meet with His people!!
Knowing that Jesus and the NT writers were torah observant should cause us to look at the NT scriptures through the same lens. By not doing so, some of the teachings are contrary to each other, ie: paul contradicts himself and others. As it is, we are looking at the NT scripture through the lens of what turns out to be the anti-semitic bias of the proto-catholic church. Mainstream christianity isn't even reading with the intent to understand any more. When it comes to anything to do with the law for christians they read with the intent to argue. Unwilling to learn from the word they learn, follow and argue for the teachings of men.
Justin martyr didn't like the jews and taught replacement theology, that gentiles who believed in Christ were the new chosen people. The NT doesn't teach this, the closest the text comes to this concept is when we read JtB say that the axe is at the base of the tree and then he calls the jews to repentance. He calls the pharisees vipers for the same reason that Jesus rebukes them...their traditions of men were making the word of God void. It was their manmade religious structures that were getting the axe...not the jewish people as the chosen people. So yeah, justin martyr was an apostate from the teaching of the apostles in the NT.