== isotropy of inertia == 
 You wrote: "In a spaceship that is moving inertially, space is seen to be isotropic for acceleration of objects: if you have, say, two marbles of equal mass with a spring in between, and you release the spring so that the marbles are ejected in opposite direction, then the two marbles will be seen to move away with the same velocity as seen from that spaceship. This is isotropy of space for acceleration: no matter in what direction you apply a force to a particular object, the amount of inertia is always the same. (By contrast, in an accelerating spaceship, inertia of objects is not isotropic with respect to the accelerating spaceship.)" 

 But I do not agree and I am getting lost, to tell the truth.  I think you may have in mind the "longitudinal relativitstic mass" and "transverse ralativistic mass" that used to be used in olden days when people were anxious to relate everything to a Newtonian framework.  (see  or Goldstein's "Classical Mechanics.")  Today, one just says that the 4-force is the derivative of the 4-momentum and usually does not fiddle with acceleration.  But, assuming you have a reason to do that, I do not agree that the experiment with the spring ejecting the marbles yields an anisotropy in the accelerated frame, at least not if the spring acts fast in relation to the spaceship's change of velocity. I mean something like that is usually a model for a decay, perhaps fission of U238.  That's usually not into two equal fragments, but it can be.  You have to be in one blazes of an accelerated frame to say that the two fission fragments do not depart with equal momenta or velocities.  If you do your experiment with a weak spring and very massive marbles, and the spaceship is accelerating very fast, you might be tempted to talk of an anisotropy of inertia or momentum, but in that case you need more sleep. You can easily detect by a variety of means that your spaceship is either accelerated or at rest in the launching dock under the influence of gravity, and you are drawing yourself into a morass by trying to introduce anisotropic inertia or mass.  When Galileo dropped things off the tower of Pisa he did not decide that inertia is anisotropic.