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However, I candidly confess that this understanding is also conjecture on my part...
What you say makes sense, and seems compatible with what the Bible teaches on a spiritual level, although the authors did not always 'get it' at the time. It seems progress and enlightenment was evident by the time of Johns writing. The Bible is full of rules, Jesus said keep every dot of the Torah (like stone adulterers and disobedient children to death? or was their a deeper meaning in what he said?) (what were Jesus commandments?) & the Apostles said abstain from blood, women submit to men, slaves submit to their masters, don't practice homosexuality etc etc.
But how do we sum up all the rules, and which do we keep? With spiritual enlightenment, in time we may understand the overriding 'royal law' which does not impose prejudices of any historic culture. (Peter, James & Paul were all wrong about some of the rules - this is clear from the Bible). If we believe the Bible as a whole teaches truth, then we can recignise that one part of the Bible supersedes others, and some authors, although inspired, were mistaken. If we believe The Bible is inspired, then why not recognise that what it really teaches is not about facts, rules, doctrines, but about revelation of the creator and relationship with God through Christ. What did Christ say was the law and the prophets? What is the overwhelming measuring rod in the New Testament? What is God? God is love.
If God intended there to be more than one gospel, perhaps it is to teach that something takes priority over all written laws and doctrines. That the truth comes from diverse sources and we must continually critically examine all human teaching. The centrality of the Bible always points towards the personal Lordship of Christ, and the glorious freedom of the spirit of the Christian man and woman.