Einstein was able to arrive at all that because he submitted his own thinking to serious constraints. He never invented new things (multilapse theory worldpretation) but actually destroyed them.

What I don't buy from his arguments though is that somehow gravitational waves would accelerate you. The universe is already accelerating, there is only the need for the waves to appear to main relativity. They don't need to be the cause of acceleration, only if you assume the universe is not accelerating.

Of course there's also the question of other reference points like looking at the stars and how they behave, but that would be too anti-dialectical for the last 300 years of philosophysical thought.