Personally, I would call 90% an overwhelming majority.
If just 10% of the population are in rebellion, a society collapses. You don't say 'the overwhelming majority of people are right handed'.
If 90% of the public agreed on an issue, the term "overwhelming majority" would probably be used in the media.
Yes, but the media are such complete idiots I could rest my case with that alone.
The way I see it, any intelligent, free-willed being has some flexibility in alignment.
Err... first of all, that's not true. There are a number of cases where we could have an intelligent, free-willed being be certainly constant in their alignment. All we have to do is assume that the creature has non-human attributes. For example, the creature might possess perfect violition - meaning that unlike me and you it never fails to do what it wants to do because some part of its being rebels and decides to watch TV instead of say studying for that exam it really wants to pass. Such a being would have a constancy of behavior that humans wouldn't have, in that it would truly always be acting to achieve the thing it wants most. Or for example, the being could be so intelligent that it always sees the end results of its actions, and hense despite having free will always chooses to do the same thing every time even knowing the outcome. Such a being never has regrets. Once its chosen something based on its desires, it always chooses that same thing. Or we might have an intelligent being that has an emotional and goal structure utterly incomprihensible to humans. For example, the being might be literally physically incapable of feeling tenderness or sympathy for anything. It's still has free will, and could choose to be nice, but it never actually desires to be nice, and so never actually chooses it. Or again, it might be phyisically impossible for a creature to become angry, as the necessary mental path ways simply didn't exist. However provoked, it might never desire to lash back. If you punched it, it might apologize for putting its face in front of your hand. If you did it again, it might realize that for some unfathomable (to it) reason you were trying to put your fist forcibly in its face and try to evade, but it would never desire to strike back in return because it lacked any emotional basis for doing so. Such a being is fully intelligent. It could explain its thinking and motivations to you, but you'd never (as a human) really be able to understand how it felt because any attempt to put yourself in its shoes would result in an error of reasoning.
Which is effectively the error I think you are making here - trying to put yourself in 'the Dragon's Shoes' as if the dragon and everything else in the universe that did or might exist worked, thinked, and felt just like you do.
But even if I can't convince you of that, consider what you are saying again with a different emphasis:
The way I see it, any intelligent, free-willed being has some flexibility in alignment.
- emphasis mine
Any in that context means 'every'. So what you are arguing is things are most logical (and presumably most interesting, since its our imagination at play here) when there is no diversity. Everything is just exactly the same, and fundamentally acts and thinks like a human regardless of shape, regardless of biology, regardless of spiritual origin.
I find that utterly unfathomable. The idea that in the full range of not only what could exist in this crazily huge universe, but in any imaginable universe, everything would be basically human in nature and demeanor strikes me as utterly unbelievable.
For a creature that is "Always Chaotic Evil" like some types of Dragon. I'd say that 90% are Chaotic Evil, 4% are Chaotic Neutral, 4% are Neutral Evil, 1.5% are either Neutral Good, True Neutral or Lawful Neutral, and 0.5% are Lawful Good.
It's an imaginary creature, so there is no wrong answer here, but I'd put those numbers too high by a factor of at least a 100 in the case of one step removed from the main alignment, and by a factor of more than a million in the case of the opposite alignment. In the case of a planeborn creature, even that level of frequency would be too high for me, but we at least seem to agree on that.