No Stupid Questions

If you're talking about self-hosted game servers, those already exists (see Minecraft), but it depends on whether they allow it or not. Reversing is possible, yeah, but it would take stupid amount of effort without their cooperation.

Main challenge of federated in the sense that there are communications between servers as well as client and server is probably going to be latency. If I were building something like this, I would rather have a protocol that redirects you to the actual game server so that you have direct connection to it rather than having your home instance acting as a proxy to the remote instance. Your home server would simply tell the remote server that your account is legitimate. This would be one of those "cracked" Minecraft server except they rely on an external server for account verification.

Garry's Mod. There is a centralized location you can all get the relevent files from, or uploaf them to (Steam). Your game could be on a publicly accessable server. But, you are able to and pretty much must host the server yourself (or you could use a hosting service, but GMod won't do it for you).

I believe Space Station 14 fits your criteria exactly if I'm interpreting it correctly. Space Station 13 would as well if not for the fact that player accounts must still be centralized to a server. That server is currently under a cyber attack, which really shows the importance of decentralization (in this instance, I'm not that big a fan of the concept).

Further edit: after reading your other comments, definitely try out either SS13 or 14. I don't play much these days because server variety has fallen off a cliff but the idea is all sorts of communities pop up with their own server which could deviate heavily from wherever they forked from. This led to all sorts of different playing experiences. Try some out to see if they fit your preferences.

Its technically doable, but it wouldn't be easy. The problem isn't in making federated user authentication or servers, its just making a game that is flexable enough with powerful enough modding tools to be worth using compared to just making a game from scratch. Thats why, if you think about all the games in the genre, they all have significant limitations to what can be made in them (at least with any amount of polish).

As a simplified example, lets say you want the default mechanics set of your game to be a first person shooter. This means you use 3D graphics, optimized for higher-detail enviroments and include features that would be most useful for that, such as a simple optimized 3D physics system, and tools for making detailed 3D levels.

In this new game, someone wants to make a 2D platformer. What additional tools will they need? A way to lock physics to two dimensions, or possibly even a new physics system entirely, A way to manipulate the camera separate from what the game started with, an orthographic camera, possibly a new lighting system, possibly different optimization techniques, the ability for the modder to replace the player controller and change it's mechanics entirely, and the tools to use all these to make new levels.

By that point, you're half way to making a new game engine, and the modder is 90% of the way to just making a standalone game.

not sure if I'm understanding, but like Minecraft servers?