If the monster targeted by “Brain Control” is flipped face-down later in the turn, it is still returned back to the original controller in the End Phase.
You cannot activate “Brain Control” if you already have 5 monsters on your side of the field.
Judge List Rulings:
If “Book of Moon” is chained to “Brain Control,” turning the target of “Brain Control” face-down, the effect of “Brain Control” would disappear and the targeted monster will remain where it is.
Related Rulings:
If the monster affected by “Change of Heart” is temporarily removed from play, that monster will return to the original controller when the monster returns to play.
A face-up “Blindly Loyal Goblin” may not be targeted by “Change of Heart.” If a face-down “Blindly Loyal Goblin” changes control through the effect of “Change of Heart” or similar cards and “Blindly Loyal Goblin” is turned face-up, control of “Blindly Loyal Goblin” will not return to the original controller.