Dear Carlos Barcelo,
Your essay contradicts to Hubble's law; outside of the Hubble's sphere the recession velocities of galaxies are greater than the speed of light. To save relativity, the physicists invented the fable as if these galaxies do not move; it is expansion of space-time only. Let's see the definition of motion: Motion - the act of changing location from one place to another. Since the superluminal galaxies changes their location concerning us, consequently it is the true MOTION. Yes, it is a FTL motion of an isolated bubble of space-time with galaxies. We cannot observe this FTL motion only because the speed of photons is limited by the speed of light c. However, the absence of direct observation of FTL motion is not a cause for the terror and persecution of FTL theories. Now all mainstream journals rejects FTL theories as ancient inquisition. In the same way inquisition pursued Bruno and Copernic because their theories contradict to official dogmas.
You wrote: In fact, not only the Alcubierre warp drive but also alternative \spacetime shortcuts", such as the Krasnikov tube [9, 10] or traversable wormholes ,seem to require the same kind of exotic matter'
There are FTL methods that don't requires any kind of exotic matter, for example Hole teleportation. If universe has a limited volume, we can teleport matter by sending an object outside of universe. What would happen if we sent a body outside the Universe? Since zero-space is a point and where time as a property does not exist, therefore it can not contain a body and consequently this body will appear in the real universe at that same moment in time. With the distances between zero-space and any other point of universe being equal to zero, these holes can potentially exist in every point of universe. Theoretically, body A could appear at random in at point of in the Universe.
For FTL teleportation of matter we need the holes in space-time only, but not exotic matter. My essay explains how to create and detect holes in space-time.