1. No. The bumps are different. A Brown's stem with a Clear's spring is a less tactile switch, not a Clear.
2. Yes. The colour of the stem is different but shaped the same.
3. Yes.
4. Good question.. Vintage unused ("new-old-stock") blues are definitely smoother, but I'm not sure about the spring.
5. Not all vintage switches of the same type have had exactly the same plastic housing throughout the years. So there are vintage blue and black switches with slightly different housing. I dunno about the same generation though.
It has been said that blue and black switches have different leaf springs inside (press-fit into the housings), which would make a blue slider and spring feel different if transplanted into a Black switch's housing with leaf springs left inside (and vice versa), but I've not tried that myself.