A difficult subject, and very much one that has been on the forefront of mind as of late. But it does seem that your quandary is actually two problems confused as one. At least, it makes more sense to me if you divide them.
This is really the first of the two problems, in my opinion, and the one that I'm still struggling with. Honestly I had to shelve Paul temporarily to reacquaint myself with the law-accepting stance articulated in the NT, which is not intended to be a rejection of Paul. Once I did, it became apparent to me that the NT writings which cater more to the early Jewish Christians place a high emphasis on the efficacy of scripture, probably including the law.
[Jas 1:21 NKJV] 21 Therefore lay aside all filthiness and overflow of wickedness, and receive with meekness the implanted word, which is able to save your souls.
[Jhn 6:68 NKJV] 68 But Simon Peter answered Him, "Lord, to whom shall we go? You have the words of eternal life."
But, if the law is applicable to Gentiles, what about the council of Jerusalem in acts 15? I think there's still something to be said about that, but also, remember it was James who presides in the text, not Paul, and this detail to me seems relevant:
[Act 15:21 NKJV] 21 "For Moses has had throughout many generations those who preach him in every city, being read in the synagogues every Sabbath."
So it seems that James, who would most likely agree with John on the use of the law in the definition of sin, and who also decides to not trouble the Gentiles who are "turning to God" with additional legal burdens beyond a few necessary things, does so while still considering the whole of the scripture to be useful, and possibly necessary, toward salvation and instruction in righteousness, etc. Of course then we have to wonder about Paul, if he really disagreed, or not. I think there is some evidence that suggests he wouldn't here:
[2Ti 3:14-17 NKJV] 14 But you must continue in the things which you have learned and been assured of, knowing from whom you have learned [them], 15 and that from childhood you have known the Holy Scriptures, which are able to make you wise for salvation through faith which is in Christ Jesus. 16 All Scripture [is] given by inspiration of God, and [is] profitable for doctrine, for reproof, for correction, for instruction in righteousness, 17 that the man of God may be complete, thoroughly equipped for every good work.
And that, to me, sounds like the same underlying doctrine at work.
Personally, I'm inclined to think of the supposed difference in the character of God to be better explained as a difference in covenants, rather than a change or difference in the character of God, which is verging on Marcion's error. The old covenant was to Paul very much a ministry of death and condemnation whereas the new was a ministry of righteousness. That law can't be binding to us otherwise we'd be required to execute vast numbers of people, which would be at cross-purposes with the Gospel, for starters.