Good points.
evolution hit on some necessary extraordinarily unlikely combination to give us intelligence and for P vs NP reasons we can't find it
For this one, you also need to explain why we can't reverse-engineer it from the human brain.
no civilization smart enough to create strong AI is stupid enough to create strong AI
This seems particularly unlikely in several ways; I'll skip the most obvious one, but also it seems unlikely that humans are "safe" in that they don't create a FOOMing AI but it wouldn't be possible even with much thought to create a strong AI that doesn't create a FOOMing successor. You may have to stop creating smarter successors at some early point in order to avoid a FOOM, but if humans can decide "we will never create a strong AI", it seems like they should also be able to decide "we'll never create a strong AI x that creates a stronger AI y that creates an even stronger AI z", and therefore be able to create an AI x' that decides "I'll never create a stronger AI y' that creates an even stronger AI z'", and then x' would be able to create a stronger AI y' that decides "I'll never create a stronger AI z''", and then y' won't be able to create any stronger successor AIs.
(Shades of the procrastination paradox.)
For this one, you also need to explain why we can't reverse-engineer it from the human brain.
It was designed by evolution. Say what you will about the blind idiot god, but it's really good at obfuscation. We could copy a human brain, and maybe even make some minor improvements, but there is no way we could ever hope to understand it.
If Strong AI turns out to not be possible, what are our best expectations today as to why?
I'm thinking of trying myself at writing a sci-fi story, do you think exploring this idea has positive utility? I'm not sure myself: it looks like the idea that intelligence explosion is a possibility could use more public exposure, as it is.
I wanted to include a popular meme image macro here, but decided against it. I can't help it: every time I think "what if", I think of this guy.