This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
The Handler doesn't have any special code for working with the GUI thread - rather it makes sure all messages are delivered to the Thread in which the Handler was created. This helps you deliver messages to the GUI thread only when you create the Handler in the GUI thread (then access it from other threads.) If you started the Handler in some other thread that was not the GUI thread you could not send messages to the GUI thread.
Knowing this, it is just a matter of using Thread.currentThread() during the initialization process to get a reference to the Thread in which the Handler was created.
Sort of. It uses a MessageQueue. Essentially the EventQueue is just a MessageQueue associated with the GUI. For a Handler created in the GUI thread you would consider it as working with the EventQueue. But the Handler works with the current Thread's Looper/MessageQueue and not all threads initially have a Looper or MessageQueue, so if you use Handler (or call Looper.prepare()) then a MessageQueue will be created for the current thread if it does not already exist. This could mean that you have multiple different threads running event-like messaging loops.