Once you upgrade and activate, a unique hash of your computer hardware profile is saved online. You won't need a key to do a clean install in the future on that hardware, just choose skip when it asks for a key. It will recognize your computer and activate automatically.
Lots of apps and even the app store a still pretty buggy. You might try doing a system refresh and see if that cures it. Hopefully they'll get all this stuff fixed up in the service release when it comes out in the next week or so.
You can do a clean install, but you must first do the upgrade so that it can verify you have an eligible license. After you have upgraded and activated you can then format the drive and do a clean install. When it asks for a key during the clean install just choose to skip it; your license information is stored online and it will recognize your computer and activate automatically. When upgrading all of your settings and personal files will be saved. The Windows.old folder should be the size of your current Windows installation. You can use disk cleanup to remove it. As for unsigned drivers this page will tell you how to use them: How to disable driver signature verification
It asks what programs you want to use as your default browser, picture viewer, etc. during setup. You can uncheck Edge and the other stuff. However they should make it more clear; you have to click a tiny little link to change them.
The glass around a light bulb is extremely necessary. It wouldn't burn very long without it. Personally this seems like a really silly, pointless device to me too. You don't need an expensive separate button to use Cortana.
Your motherboard and its components primarily (NIC, CPU, etc), and then I think things like your RAM, video card, and so on. It takes all of these parts and generates a unique identifier for your PC. You can usually change several components without any problems, like a new hard drive or video card. With XP through 8 changing your motherboard or too many other components would cause you to have to reactivate; sometimes you would have to do so by phone. I'm not sure yet how it will work with Windows 10 as far as upgrading components goes.
You won't need a key to reinstall or do a clean install. As I understand it Windows 10 creates a unique hash for your hardware during the upgrade and stores it online. The next time you install Windows 10 on that hardware it will look up this info, recognize your machine and install without any need for keys.