As far as I know, both Asakura and Yuki were designed that way. Kyon said something to that effect I believe.
I would disagree on other people being white boxes to us. If you have ever been in a relationship this you would know very well
You never know what your significant other (or someone you are interested in) is thinking
Again, I would not conclude that Yuki, or the humanoid interfaces are incapable of emotion. I'm not saying that you are wrong, just that we don't know enough to conclude either way.