To sum it up then only US can do anything relevant?
BTW they could have invaded Spain, sure, but that doesn´t mean they HAVE to send armies there to push the allies back.
Yup, but then they would have had a solid base from which to advance into France, at which point the Axis would still have had to respond. Like I said, it would have worked out better than what did happen.
Invasions of other areas (Balkans, Italy) wouldn't really offer the same prospect although they would stop the Axis from being able to concentrate their forces.