I guess the difficulty I have is that what you describe aligns pretty closely with, e.g., Hermione's arc and characterization, too; so it's evidently possible for one to be a misfit geek and analytical while not having those be one's defining attributes (by the weird and oversimplistic yardstick of a magic hat, I mean). Though it's probably fair to say that Hermione is in Gryffindor as much because it was easier for the author to set up in-House friendships as because she belonged there, much as with the personalities of the various Marauders a generation earlier. I think part of the thing with Aral and Cordelia and such is also that, because we do see them as adults first and only get a minimal sense of their difficult adolescences, it's tempting to look at their learned skills and habits over their root natures. Though even as an eleven year old I would argue that Cordelia was probably not a little Hufflepuffish; brilliant and inquisitive, sure, but also very much dedicated, and loyal to a fault. Her social skills are learned, but what Aral terms her ability to carry around civilization with her-- that instinctual sense of duty-- is not.

I feel like Miles wants to be the rescuer as much as he wants to give everyone a rescue, which is why, while I agree that he probably ends up a Gryffindor, I don't think it's all that clear-cut. Even as an adult, post-Admiral Naismith and settled into his role as Lord Vorkosigan, his first response to hearing that someone else has saved to day is to think "Well, I could have done so just as well!" He then goes back and acknowledges that the thought is unworthy, because he is an adult and he's grown a lot from his seventeen-year-old-heroing, but it's because he's grown away from that knight errant strain that he can moderate his own competitiveness and desire for glory. Though now he's starting to sound like James Potter, so I may have talked myself into a corner here.

As for why people in fandom are Slytherin-happy, speaking for myself, I think part of it is the fact that Sorting is such a terrible idea, and ends up with things like a quarter of the population being deemed ubiquitously untrustworthy: the contrary streak in me makes me automatically want to see if I can't apply the description of Slytherin to heroic types. But yeah, as an actual thing to do to real people, and children at that-- completely toxic.

(I really like the Alys-Piotr parallel. I can only imagine what she would have done to the Cetagandans.)