Sorin's +1 can be confusing for many people. How exactly do you determine which creatures get the bonus? Well, the creatures that will be affected are locked in at the moment the ability resolves, and that bonus simply lasts on those creatures until Sorin's controller's next turn begins.
Heart-Piercer Bow has its own triggered ability, which is controlled by the player who controls the Bow, no matter who controls the creature its attached to. This can be a little confusing with Act of Treason, so read up on that interaction here!
Meandering Towershell's trigger uses the wording "your control" which is distinctly different from "it's owner's control." That means it's pretty easy to take control of the Turtle permanently with thinks like Act of Treason!
There are plenty of ways to 'steal' permanents in Magic. When a permanent does change controllers it is the same object that was previously in play. If it is a creature it will not be able to attack the turn it is 'stolen' unless it has haste or the effects that causes it to change controllers gives it haste. However sometimes even if the creature has haste that won't work. As this permanent is the same object it will still be tapped or untapped. As with granting haste, some change of control