

This is different, by design. Sideloading and alternate app stores are still perfectly fine. It’s just that the developer has to give a bunch of personal into to Google in order for their app not to be rejected by the phone itself. Middleman doesn’t matter which is where most if the regulations are. Problem is that many open source apps don’t have a developer willing to spend the time and money or give up their privacy in order to publish an app they don’t make money on. So it’s designed to kill off FOSS apps.
Possibly, but I don’t see Google doing that either. It’s not about where the app comes from really. The thing they are going to restrict is the developer. A developer can not have their app installed on a certified phone, regardless of where the user got the app, if the developer is not registered.
So, since there are no regulations to allow a user to install apps from any chosen developer, only from any chosen app store, there likely will be no regulatory recourse like Apple is facing as mentioned by the OP.