Keplerian Space Discovery, a more serious take on the KSP-like realistic spaceship simulator formula, released in early access with an ambitious roadmap

No idea, as I haven't used unreal engine, but considering how the concept of scenes and actors don't really make sense in a space game, at least on this scale, I think Rocketwerkz' approach of building from scratch is at least an understandable approach. Especially when performance comes into play.

That'd be totally unrelated to the engine. Regular physics engines don't handle that kind of stuff. They only handle collision. It's up to the studio to implement gravity, and it's they're decision on how that's done. There's no reason n-body physics couldn't be supported. It can be in KSP even. It's just significantly more calculations to perform, hurting performance, for a very insignificant (usually) effect. It's also much harder to play, teach, and learn. I would say a smart studio doesn't do n-body by default, but maybe create an option for it, though this requires a totally separate gravity implementation.