As I understand it, the delivery mechanism won't matter: Play Store,ADB, F-Droid, Bluetooth, or website. If the APK isn't signed by a Google-approved developer, it's not going to install.
If there's some ADB command that one can issue to install unsigned APKs for now, it's a temporary reprieve at best. Two Android versions later, the update from Google will read "Only 0.02% of users installed apps using adb, but the corresponding malware incidence rate was 873% more than the Play Store. Due to the outsized risk, we're disabling adb installations going forward"
No, that adb command is how you test install things. They wouldn't want to force public uploads to Play just to test.
Not so. The new mandate isn't that all APKs must be uploaded anywhere, only that all APKs must be signed by approved developer keys. So to test new builds, devs will only have to sign with their approved key, then upload. No extra hassle once you already have an approved key.
I'm not sure it works that way. _In general_ before the recent announcement you are supposed to sign the debug build (what you feed into adb to install) with your debug key that's different from the release nor upload key, and the debug key is never submitted to google.
Of course _maybe_ at some point google will also force you to submit your debug key to them. But I don't believe that's the case now.
Sure, you would test-install apps via any delivery method of your choice, including USB-C cable or WiFi, after Google attests that your test-app signature is whitelised[0]. After all, there is no legitimate reason[1] to not sign your app, since you want it to closely match the distributed version as much as possible, and there won't exist unsigned distributable apps.
0. Developer has valid signatures and in Google's good graces, and application hasn't been installed on more than 16 devices
1. Oh, you CI/CD signing infra won't let you? You better fix your workflows to match the Google way.
They could go the apple way and sign an annoyingly shortlived cert.
Won't fly given the existence of tons of playless Android forks that F-Droid or other methods can easily be deployed to.