If I were to theoretically sell my game code, how would I do that? Would that be possible?

I’m curious to know if I could create a game using Construct 3 and then sell it to a different developer who doesn’t use Construct 3 as an engine.

You wouldn't be able to - the engine is integral to how the game works; even one entirely powered by JS would refer to elements populated by the IDE

No, but then again the reverse would also be no.

Nothing works like that.