Spelljammer Void Worlds

[Khedrac]

(3) nothing prevents two objects from being subject to each other's gravity

And this is where your argument falls over because in Spelljammer #3 is incorrect. This is most noticable when a small spelljammer nears a planet or a large spelljammer, the smaller spelljammer's gravity plane is overridden and the larger object's gravity applies. This in no way works in reverse (it's used as a combat tactic by big ships) - only the dominant gravity effect applies.