1. there are things called gnome extensions that change things up.
2. it’s just that a lot of laptops are potatoes with wierd hardware and drivers aren’t always available. If you have a popular laptop you’ll have better luck. Can’t predict how it’ll go other than goggling your laptop and seeing if you can find a post saying what worked and didn’t. Can’t hurt to try either way…
3. yes. There are plenty with installed apps. Hard to believe you didn’t find any music or video players. Either way - doesn’t matter. Install VLC and it plays everything.
4. most Linux distributions will let you delete Linux itself if you’re so inclined. My vote is to just leave the default programs that install with the distro unless you’re in need of an absolute bare bones system/size (which it doesn’t sound like you are)
5. root is a user, nothing more. If you don’t know why you’re using root, then don’t. Based on your questions, I’d say you can do everything you need as a normal user with sudo privileges.
6. to be honest I’ve never actually done this. I believe you can even install multiples at once and switch between them. Most distros come with a choice of DE during install. Check them out in a vm and just install the one you want. If you’re hell bent on swapping on an existing install, best read a guide on how to do it for your distro.
7. this isn’t exactly right, but docker is kind of like virtual machines. Not quite full on VMs, but rather they are called containers. You can download a docker image, and fire up say, a pihole server. Or in my case, I run a preconfigured ubiquity WiFi controller. Don’t worry about these for now - it’s a later thing. Wayland is replacing X. Some distros use it, some don’t. X is very old - it’s stable and doesn’t get updates and just works. Until it doesn’t because it’s old and doesn’t get updates. Enter Wayland. New things of that complexity are hard to make so there’s bugs with it. Works for some people, not for others. Go watch some YouTube videos on the topic - it’s interesting.

Good luck!