I didn't vote one way or the other on your original query, but in reading it, I would likely have voted to close for being story based. This is why:
- You mention the centrality of narrative no fewer than four times in your question. In fact, you introduce your question In the near future Science Fiction novel I am writing; which is followed up by narrative necessity and plot hole and In my story. These phrases indicate to us that you're having a writing difficulty of some kind rather than a worldbuilding issue per se. Especially narrative necessity: when you admit to that, you are very squarely focusing your question on matters of plot rather than nature of the world itself.
- WB.SE doesn't accept queries about plot holes or how to arrange the narrative elements or a story. Nor does it help people with issues of narrative necessity.
- Our purpose is to help worldbuilders, and even (would-be) authors such as yourself!, with the fundamental systems and functionality of their worlds.
As I read your query, you've already got a good hold on point 3, so there's really nothing for us to answer. Also, for points 1 & 2, you've already answered yourself to my satisfaction: in this instance, there must have been some kind of fatal error (some kind of trauma perhaps) that would preclude implantation of the original brain. I don't see any point in going further along those lines.
So, what can be done about this situation?
Well, the main thing is to edit your question so that it is no longer about the story you're writing. You mention medical, technical, ethical issues in passing as potential lines of enquiry and you tagged your question "science based". I think if you were to remove all mention of your story as irrelevant (we assume that people either make invented worlds for fun or they make them in which to set stories); leave in the background data on what's possible in your world; and then focus on what the real problem is, you'll have at least one and perhaps two related but much more SE friendly queries.
The medical morality & ethics line of enquiry would be fascinating to explore in an answer. Medical technical issues would also be quite interesting. Plus I can show pictures of damaged brains and surgeries. ;)