I like a lot the spirit of your post(s), just one thing
Dei Verbum, for example, acknowledges that there are “seeds of the Word” in other religions. I’ve often used terms like Kali Yuga myself (because, frankly, it describes the modern world perfectly) despite it not belonging to the Christian tradition. But none of this means that the principle of non-contradiction can simply be thrown out the window.
Take reincarnation, for instance. It stands in direct opposition to core Christian beliefs.
First, it effectively sidelines the redemptive sacrifice of Christ, since salvation becomes something you achieve yourself over multiple lives (assuming there even is a “you” to speak of, which is already questionable in this doctrine; but we’ll get back to it).
Second, it undermines personal identity altogether. If you remember nothing of your alleged past lives, in what meaningful sense are you the same person or even the same entity in a broader way (so to speak)? At that point, we’re not talking about survival of the self, but about its practical annihilation under a different label. And the “new you” is a completely, absolutely new person who has basically nothing to do with the person(s) “you” (I’m using “you” even though it doesn’t make any sense in this context as there is no lasting “you” at all) were in previous lives. It’s basically, for the person you are now, the same as secular annihilation, the practical implications are the same, for your current identity.
It’s also telling that many atheists who aren’t strict materialists (there are some atheists who aren’t hardcore materialists even though they are very few and far between) seem far more open to reincarnation than to Christianity. It’s almost as if the idea of a lasting personal identity is what they really struggle with. Sometimes it even seems like they find it unsettling ( as though the prospect of truly encountering again the people you love were somehow undesirable. One has to wonder whether they’ve really thought through the implications of what they think, or whether they even really love someone else, because when you love someone, you simply cannot want them to disappear forever, nor can you be content with it. You might believe, as a materialist, that they will indeed cease to exist, but if you truly love them, you cannot honestly be at peace with that at all).
In short, I agree that seeds of truth can be found in different traditions, but only up to a point. Some claims are simply incompatible, and when that happens, they cannot all be true at the same time.