In answer to your simple question:
Two sleep clause questions:
1) If I encore someone into spore, and they put two of my Pokémon to sleep by staying in have they violated sleep clause?
It presents an interesting situation but ultimately yes your opponent would be breaking sleep clause, and either A) You could call for a disqualify or B) You could wait till the second Pokemon wakes up, and try and set up the whole situation again(only instead of sporing again he would switch as well).
2) If one of the opponent's Pokémon is already asleep, then it tries to put one of mine to sleep, but I predict this and switch out into a Pokémon with magic mirror and reflect it, have I violated sleep clause or did my opponent put himself to sleep?
Nope you didn't use a sleep inducing move, its much like if you spore one of his pokemon and then he uses rest. 2 of his pokemon are asleep but it does not violate sleep clause.