Thanks for your answer!!!
'assuming that it is wrong for him to keep the Law in any case' - that phrasing can be easily misunderstood. Paul appears in some parts of his writings to teach the law i.e the commandments, but there are other aspects to it such as the Jewish customs. James equates a Jew who does not do the customs as apostatising from Moses in Acts 21. In the same passage he solidifies the sentiment that the kosher laws to gentiles are actually a part of the Law, hence they were coined as 'requirements' in Acts 15. (Paul agreed to this initially in Acts 15, but went away to teach otherwise).
It doesn't matter about the 'voluntary' nature, the fact is that the underlying meaning/value/purpose of the vow no longer holds. That's why Christians have had to say this is but a tribute act to Jesus's sacrifice.
With regards to your second point, that's a disturbing position to hold as it implies deception, which in this context involves the many thousands of the newbie believing Jews who obviously didn't ALL have the right understanding of Law. The problem is exacerbated when James actually speaks of them being zealous in a way in which clearly shows James is proud of it. What we take from this as, because James selected a nazarite vow (out of all of the other customs, he had to go for the expensive one which also should no longer have the same value to the new believers), it's clear James wants Paul to prove he is obedient to the Law to these zealous guys, thus affirming the sentiments of the vow. James then, was deceived by Paul, too.
Why the nazarite vow, that's littered with all of these customs which are full of conditions that you must stick to in order to become purified otherwise you cannot do certain acts of worship because you are unclean - these things have no salvific value in Paul's doctrine. Also, it has a sin-offering in it, too. If it's voluntary, why are James and the new believers under the vow choosing to do something like this vow in the first place? It's clear they uphold it's Torah meaning, they continued to do them.
With regards to your fourth point, there was a 'sin-offering' to conclude nazarite vows. This is the sort of mode created for those under the law. But they go on to affirm the nazarite vow and with it there is a sin offering involved. This is completely against what Paul's doctrine is about, but if he were to stand up against doing it, he would have the disciples brand him an apostate.
Fifth point, again that raises the problem of deception.
Sixth, were they really false? At the end of the day, Paul did speak of the Law in a universal way.
Remember that Paul didn't really meet with Peter, James and John much at all.
Verses 20-25 record the advice of the leaders of the Church of Jerusalem to Paul, beginning with their reaction to Paul's report in verse 20a: And they, when they heard it, glorified God. This showed that the leaders of the Church of Jerusalem were happy over so many Gentiles coming to a saving faith.
They then related to Paul the state of the Church of Jerusalem in verse 20b: and they said unto him, You see, brother, how many thousands there are among the Jews of them that have believed; and they are all zealous for the law. The elders refer to Paul as brother, showing they viewed him as a fellow believer in the ministry, and they point out: how many thousands there are among the Jews.
The Greek word for thousands is the word from which the English word “myriads” comes. What James and the elders said was, “Notice how many myriads of the Jews have become believers.” A single myriad is ten thousand, but James used a plural, meaning “tens of thousands.” There was a minimum of twenty-thousand Jewish believers in the Jerusalem Church alone; it had become a sizable movement. These myriads of Jewish believers were all zealous for the law.
The Greek word does not mean zealous as an adjective, but Zealots. The Jerusalem Church was largely committed to keeping the Law of Moses. This was no longer mandatory , so why did they keep it? They kept it for one of two possible reasons. First, they may have done it as a free option, for freedom from the Law also means one is free to keep it if he so desires.
But secondly , and more likely as known from the overall context, they were still spiritually immature, and, in their immaturity , they still felt they were obligated to keep the Law. But they were not sinning by keeping the Law, they were only wrong in making it mandatory . The Jerusalem Church was composed of Jews who had chosen to keep the Law whether it was for the first or the second reason.