What happens to a permanent I gained control over using Agent of Treachery, and I leave a multiplayer game?
When a player leaves the game, all objects (see rule 109) owned by that player leave the game, any effects which give that player control of any objects or players end, and all spells and abilities controlled by that player on the stack cease to exist.


(emphasis mine)

So Alice's control of the Leyline ends, and Bob controls it again. The Leyline won't be affected by


  Then, if there are any objects still controlled by that player, those objects are exiled.


This sentence rarely matters, but as @Hackworth notes in the comments, Command the Dreadhorde is one of those situations. The Rulings under that card mention it as well:


  In a multiplayer game, if a player leaves the game, all cards that player owns leave as well. If you leave the game, the creatures and planeswalkers you control from Command the Dreadhorde’s effect are exiled.




Regarding Beguiler of Wills, you can think of the Rulings in Gatherer as a sort of FAQ about the card. All Rulings can be traced back to the Comprehensive Rules; apparently Wizards didn't feel the need to add it. The situation is the same as with Agent of Treachery; the 'stolen' creature won't be exiled but will once again be controlled by its owner.