Kinetic energy is relative to some inertial reference frame. You can pick any reference frame you want and each will give you different numbers, but conservation laws should still hold in each.

Addition of kinetic energy is pretty unintuitive in this regard, e.g. when throwing a ball, the "work" done by your muscles depends on the reference frame that you pick to analyze the event.

In empty space yes, but space is a vacuum (mostly) but not empty.

If you are in orbit you can measure your speed relative to something else, the only that doesn't work is a privileged frame.