They must release the source to anyone they give a copy of the binaries, which - since the software is free - is anyone who goes to their website and clicks "download."
It's not as simple as "if they ask" - if you choose not to distribute the source code with the binary or provide it for download, you must explicitly provide a written offer valid for 3 years for the source code.
I wouldn't think so (you'd need wording like "I agree to distribute source code to you on request to <address>"), but I doubt it's been challenged - it's an obscure use case of the GPL.
Not that obscure; lots of hardware vendors use Linux in their routers/TVs/tablets/phones/etc and distribute the sources separately. In fact, every manufacturer of Android phones should have such an offer included with their device.