To further your point, I like to imagine that whatever shows up on Earth is gonna be something like a von Neumann probe. Self replicating, probably AI.
That would account for the vast distances a biological body couldn't travel, as well as the violent direction changes you're talking about.
But arguments against alien craft that question why they move so quickly with abrupt unnecessary direction changes, and lack of interaction with humans, to me could be explained by imagining that they are “unmanned” craft. No need for “inertial dampers” or other tech to deal with the hyper fast direction changes if there’s no beings inside.