The series implies that Mars is "on the way to Earth." For more than six months out of the year, Mars is on the opposite side of the Sun from Earth. Any ships coming to Earth would be able to travel there from nearly any direction (or if they use a jump gate, as is shown in the series, from a specific single direction) so there would be no reason to travel anywhere near Mars.
Multiple people state that the Vorlon's quarters contain a lethal combination of gases that would kill humans. However, no one decontaminates or changes their uniform immediately after they leave. No one even wears gloves inside, even though toxic gases could settle upon their skin, or be absorbed through it.