...and foil power would be (at least I assume i would) with new corrected text and new card layout. Which, in my opinion, would be lame.   A lotus that looks like a Kamigawa card just wouldn't be that nice.

Do you mean that when you're Slavering your opponent, you cast a Control Magic? To make it a short answer, the opponent controls the creature. (It doesn't matter whether you played it on your creature or the opponent's)

This is simply because "you" in the spell always means the caster, and in this case your opponent played it, while being under your sovereign control. Compare this to making your opponent to play a Vampiric tutor, for example. He loses 2 life, not you.