Agree with your overall take. I found Charlie Cox's almost green boyishness rather powerful, in contrast to Hiddleston's settled urbanity. It was part class distinction and part temperament, since the tamped down Robert -- attractive though he surely is -- feels cerebral. Cox had that almost post-adolescent sensibility and athleticism.
The slow circle with the child was riveting to me, and (maybe too obvious) made me think of Andy Blankenbuehler's work. The pace of the movement was what gave the image distinction, a tableau semi-vivant.
I wasn't much interested in another "Betrayal," and was surprised by how much this one made me lean forward to listen. I have discovered that those of us who've seen other iterations are more taken with this staging. A friend who'd read the play but never seen it thought this underwhelming, even disappointing.