The post above spooked me into calling MPC, worried about the "alternative flights". The center was very helpful. I cleaned-up some alternates that had cleared that I was not going to use. But I did keep one. They didn't mention it. Perhaps they didn't notice it. I certainly wasn't going to call attention to it.
And to answer my original question, I was told that altering the other segments did not impact your position in the waitlist for the remaining segments. "First come, first served" was the exact phrase she used.