My point is more that expansion of the strong AI would not occur at the speed of light
How do you know that? Or why do you think it's a reasonable assumption?
so there should be very distant but observable galactic-level civilizations of AIs changing the very nature of the regions they reside in, in ways that would be spectrally observable.
How would we tell if a phenomenon is natural or artificial?
in those multiverses where a local AI respects some sort of prime directive, we may be left alone our immediate stellar neighborhood should nevertheless contain signs of extraterrestrial resource usage.
It would not be a good implementation of the prime directive if the signs of superior intelligences were obvious.
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.