Hey @phil,
Unfortunately, often, to learn one needs to suffer. There's one thing I hope you learned from this, which from the way you describe seemed painful, is that AI chatbots are not what they claim to be. Another thing I think would be worth learning is that before using something one should learn a bit what that thing is, who made it and for what purpose, it's wiser in order not to get into trouble. An AI chatbot will just tell you what you want to hear and repeat what most people would tell you (or better what people said most in it's training data). Without enough training data on an issue a chatbot will just invent whatever random series of word seem could fit your question. So I'm not surprised it lead you into infinite complications.
About grav, I think pretty much each and every question that you've asked and that Karmalakas answered very patiently is actually also answered just by reading one single page: the login plugin documentation.
For your last questions on pages you want private not being private, my intuition is that you didn't read the docs, especially the part about how to make a page private.
And to me it makes sense that a website has both public and private page, and only the site admin / authorized users can know which page should be public and which not, so by default I don't see why pages should magically become private when you install the plugin.
That said, personally, without actually seeing your configuration I'm not able to provide any more help because I don't have enough theoretic knowledge of the plugin, it's actually reading the doc and trying to apply what it says that I think fiddling with the config one can fully figure out how to use it according to it's needs.
But I'll stop commenting because I think I'm not really bringing valuable info to this forum, and I'd prefer contribute with real knowledge or experience I have.
Wish you the best of luck for what you're trying to achieve!