While you can’t uninstall Safari, it doesn’t constantly discourage you to use other browsers like Edge does. Nor does Mac OS prevents you from installing competing apps.
The bigger problem is iOS, but the EU already took care of that and we’ll be able to sideload apps on iOS pretty soon.
App association is done at the OS level, and the apps are normally responsible for that. So it could be either the OS not registering the selected browser properly or the other browser not registering itself correctly as the default browser.
They need to basically register themselves as responsible for html files and a bunch of protocols (http, https, etc). I’ve never had a problem like that, and I’ve been using macs for almost 30 years (I’ve used many different browsers as default in the past).
But browsers are pretty complicated beasts, so I believe you. There are a lot of things that can go wrong and your choice may not end up being respected.
No, you're confusing MacOS with iOS. Mac allows whatever you want. Each browser has its own rendering engine. iOS is the one that only allows (for the moment) Webkit. But that's going to change (at least in Europe). Here: https://en.wikipedia.org/wiki/Comparison_of_browser_engines#Support
Older MacOS versions had stuff like the chess game preinstalled for no reason, though I don't know how current versions look like.
I also don't know how easy it is to remove preinstalled apps nowadays. Back in the day, you could disable System Integrity Protection, remove whatever you want, and re-enable Protection afterwards.
That chess game even predates OS X, it was a tech demo that came with the NextStep OS and has barely changed since the mid nineties. At this point it would be said to see it go.