I am not a theologian but have had a rather broad experential exposure to various groups of Christian thought so I’ll give it a shot.
But first there needs to be a more precise clarification of terminology. I understand what you are saying about the Gnostic way of thinking which you expressed. However, this differs from the viewpoint of mainstream Christianity which sees mankind as having three distinct parts in one. Body, soul and spirit.
Body is obvious. Soul is regarded as basically the combination of mind, emotions (psyche) and the will (volition). The essence of ones personality which makes them separate and distinct from anyone else. Even identical twins have totally distinct personalities, one from the other.
In my years of teaching Elementary, I’ve encountered several sets of twins and even if they look exactly alike, physically speaking, it doesn’t take more than a few days to be able to differentiate them based upon their personality and manner of interaction with you.
Unless a teacher has very little interaction with them (as in college lecture halls) the idea of them being able to exchange between themselves and fool someone is ridiculous.
Perhaps for 5mins or so but that’s it unless the person dealing with either one is an oblivious dunderhead. Two distinct souls regardless of the identical DNA. definitely not identical personalities.
That leaves the spirit which is both the initial breath of God and the ongoing force which males it possible to commune with, and eventually unite with Him in a spiritual dimension which is on a higher level and more profound than the mere mind or emotions.
That’s nothing new. It’s fairly straightforward standard theology common to most mainstream Christian groups.
There are numerous Biblical references which form the core of that framework. Far too many for me to enumerate.
However, if you’d really like it further elucidated, there’s a fairly short book, THE RELEASE OF THE SPIRIT written by Watchman Nee which gives a pretty clear breakdown of each of those terms along with plenty of scriptural references.
He was a highly respected Bible teacher and house-church planter in China. The rise of numerous local house churches kept Christianity alive and even thriving during the worst of the Communist repression.
Or you can go to the following website which outlines a lot of the same ideas.
www.tripartiteman.org
You’ll find more scripture references than you could ever hope for if you are truly interested in the subject in depth.
For the second part of your question, in which I’m assuming you’re throwing down the gauntlet by presentimg these examples of fairly rare anomalous circumstances, there really isn’t a “one-size-fits-all” answer.
The primary reason being that you are making the assumption (wrongly) that all Christian theology holds that life (and the combination of spirit/soul) begins at conception.
That is primarily the viewpoint held by the RC Church and most members of the ardently militant “right to life” groups. One of their favorite verses is from the Old Testament, and to paraphrase from memory “when you were formed in the womb, I knew you” (I’ll include the specific chapter and verse at the end of this post when I have the chance to find it)
However, that position about a specific life (identity) beginning (and being a completed soul/spirit) at the exact moment of conception is not a position universally held by every Christian group.
Most have a much more nuanced approach. Some hold that it’s at the moment when the child is born, takes its first breath (the breath of life given by God, etc.)
Others just state that it may be anywhere between conception to the first trimester to seconds before or after birth. We just cant know for certain They feel that there is not enough Biblical clarity to make a definitive pronouncement.
Their basic position on this issue (as well as a number of other issues) is that where the Scriptures do not speak clearly, we dare not insist upon substituting our own ideas.
To me, personally, that’s a far more intellectually (and spiritually) honest position to take, even tho it leaves lovers of dogma dissatisfied.
To state flatly that at the moment of conception, when as little as two cells are connected, that its a life with a complete soul and is pretty untenable to support with the type of certainty proclaimed by some groups.
If you really want to know from which specific Bible verses they draw their conclusions (and aren’t just looking to start an argument in order to prove the intellectual superiority of Atheism) you can go to any of the numerous right to life groups or RC dogma pages.
I have a general overview of their viewpoint but would make for a very poor apologist for their position since I don’t agree with their interpretation of whatever verses they use.
But since all Christians don’t think alike on this complex subject, the whole issue of resorbed twins and Chimeras (so extremely rare) are basically moot as far as I’m concerned.
If the spirit does not enter until actual birth, then what happened prior doesn’t really factor in. And even the Chimera is basically a non-issue since it’s manifestation of conflict is physical and any problems with it limited to the medical field (not the mind, emotions, will, or psychiatry or spiritual life).
Chimera is a body issue, not a spirit or soul issue. Otherwise we could drag this out to asking if MPD (multiple personality disorder) or Schizophrenia produces more than one spirit.
So this could get as ridiculous as one cares to make it. Might as well argue about how many angels can fit on the head of a pin or try to figure out if God could make a rock so heavy that even He can’t move it. I’m sure you’re aware that these and other circular arguments have been discussed to no avail.
I’m pretty sure that those who hold to the dogma about the spirit beginning at the moment of conception would be little swayed by the distractions of resorbed twins and Chimeras even if they knew what that meant. They would simply declare it a “mystery” and that would be the end of the discussion.
The scriptures upon which their assertions are based are easily found on any number of sites for anyone genuinely interested.