It was at that point I was was convinced that emailing legal to ask permission was a hack to get you to disclose early what you were working on so that when they sued you they already had some of the initial leg work done :-(.
I consulted with an attorney on this and they advised me that because I was working in California the burden of proof that it was related to Google's business was on Google. But that in his experience they were not above pushing the edge of the definition. The bigger point though was that people don't work at companies that are suing them and generally don't work at any company if they have a reputation (real or imagined) of doing work on their own time that is "against" their employers interest.
The summary of his advice was never, under any circumstances, work on any project you don't want to give Google ownership of while you are employed there. And when you have an idea that you want to develop, quit so that those parts of your employment agreement are nullified.
They can still come after you if they think you are using "proprietary or confidential" information in your new effort but that is less common and it makes them look like the bad guys not you.
TL;DR If you could not have built the product without being an employee of the company, it probably belongs to them.
eg. Maybe you use Microsoft's latest internal only HoloLens prototype to build a product. You couldn't have done that if you weren't a MS employee - so they can claim it. But if you used the dev kit and publicly available tech and features then they shouldn't be able to claim it.
If I use an internal prototype they have claim.
Sorry if I wasn't clear enough before.
Companies like Google, Amazon, and Microsoft have arms for hardware design, video game development, web development, close-to-the-metal graphical programming, embedded systems, retail...that's not just big enough to drive a truck through, it's big enough for that platform they truck space shuttles around on!
You can say that those companies are unlikely to realistically claim ownership of anything that YOU didn't work closely with, but I can say from experience that these companies have explicit company-wide policies regarding some of those areas, with stricter mandatory reporting guidelines, and even requirements to use that company's technology, separate from their ordinary side project/open source contribution/etc. policies.
So I wholeheartedly agree with your second point - what you do off the clock is none of your employer's business, full stop. Just don't use company equipment for it.