Don't call it "sideloading" it's a negative term attempting to rebrand installing any app you want: taboo and illegal by effect. Call it what it is, installing apps. Thats it.

Not at all, they can still require apps to be signed, so long as you're free to allow app installations. They'll weasel out by saying the signing is for safety/security reasons, to avoid malware and shit.

To be clear, it's not the Supreme Court's order, they only declined the appeal or to issue a stay, so technically it is a "regular" court order, which is just as binding.

Google's announced plans aren't actually about restricting "sideloading" (aka installing). They're about restricting actually running the code, and developing/distributing the code.

Content distributed through these "other app stores" and elsewhere will remain installable, but your phone will refuse to run the software if that content isn't signed with the special Google's Favorite Boy token that the developer got by providing personally identifiable information to Google. Get that "certification"? Your app can be run on Android no matter how you distribute it, including "side loading" (download APK file, install app).

Don't have the Official Nintendo Seal of Quality Verified Developer token to sign your app with? The only way a user can run your code is by connecting to a PC and installing your app with adb (a development tool).