When they were young, and first married, Elizabeth was certainly the greatest force in their lives. This pretty much continued throughout all the years of their marriage. Perhaps without the pressure she put on them to get along, and especially to have a heir, their marriage might have worked better. Yet, still Peter was clueless and not all there, while Catherine was more intelligent than many women before or since in her position.
As soon as Elizabeth was gone, Peter and Catherine both took control of their own destinies, which fitted because they were such different people. In the end, I feel that Elizabeth would have been okay with what Catherine did, yet during her lifetime appearances of who was heir, and everything had to kept up, with little way of saying Catherine really was much more suited to rule Russian than Peter. Maybe in the back of her mind, it would not have surprised Elizabeth, what actually happened.