AFAIK, there is such a thing as a CMB 'rest frame' (the Earth-Sun is moving relative to it at about 600km/s). As for the weird distances, just imagine an ant m

AFAIK, there is such a thing as a CMB 'rest frame' (the Earth-Sun is moving relative to it at about 600km/s). As for the weird distances, just imagine an ant moving across a rubber surface at constant velocity. The rubber is slowly stretched over time, so while the ant continues at the same rate it will actually cover less space. If the rubber continues to stretch, there is an event horizon effect, meaning some distant parts of the rubber sheet will become inaccessible.