Thanks everyone for your thoughts. I think I am going to stick with the seed stitch because it matches the centers of the large cables. I am going to go down a needle size. The yarn I'm using isn't very stretchy so I think that's part of my issue. (Bernat Satin--let me tell you, it does hold up to repeated frogging very well!)
I am also thinking about changing the twists on the outside of the big cable to something else--it's really clever how the designer used them to create the illusion of one big cable, but it just doesn't look very good, and I've tried executing them in a couple of different ways. I think it's again related to the non-stretchiness of the yarn.
Hopefully this most recent frogging will be the last!