NO!
eevee can only evolve once but if you get another eevee (of different gender )and breed with it you can get another eevee and evolve it into something else.
no, you have to trade, everybody who tells you something different is lying
No, it wont.
maybe they breathed in something different
It depends, different Pokemon evolve in many different ways.
Different Pokemon evolve at different levels
If you have made your eevee evolve and still need to get data on a different evolution the easiest way is to breed the eevee evolution (Vaporeon for example) the resulting egg should hatch an eevee that you can evolve into something different.
I'm not entirely sure but I think seviper,but you have to do something to get it to evolve it might be levels,or a stone,happiness,a move,or something.
after your Pokemon is at that level or you give something to it, press evolve
Murkrow doesn't just evolve it is needed something special
Theoretically it is possible, but "theoretical" and "probable" are two very, very different things.
This question is too vague. There are lots of different Pokemon in Sapphire. They evolve at different levels and in fact, some of them don't ever evolve.