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Well, if you mean does the enemy Pokémon have to faint, then yes, of course it needs to faint. XD But if you're asking if your Pokémon will still get EVs if you send it out, then switch it out immediately, then yes, it will get the EVs as long as it was out in battle at some point.
That's rather situational, and really depends on what Pokémon you are raising. Some Pokémon have high enough base stats that it would be better to raise where it is low, while others, you will want to have their high stats as high as possible.
Well, I guess you can fill up Hitmochan's Attack and Defense, since it's defense is hard to take down, and most rock are slower and are kinda' weak against Hitmochans def.