> when space is stretched dimensionless particles traveling through that space are not stretched, because they are dimensionless. Or perhaps better, objects in space stretch, but their momentum vectors don't stretch.

This is an interesting idea, but I think it's not right.

To be honest, even as an experimentalist who worked on LIGO, I'm not sure how to treat the effect of the g.w. on photons in the arm. Instead I think of the entire arm cavity vacuum as a giant phase modulator.