Thanks Tom, you set me on a good solution path! I was able to select the already edited, expanded audio elements in groups and batch change them. Fortunately, I had only 30 minutes already done out of a 2+ hour long project, so hopefully the remaining will now be easier! I still think this is not totally intuitive and I haven't found much info about this in the user manual.
I knew about the Pan-law concept in the context of a mixing console, but not under that name! I find it strange that FCP-X would use this on my tracks as I have not used the pan function. I have a stereo element that stays as-is and two mono files assigned hard Left and Right, all of them with the Pan mode set to None.
I took written notes about all this to not forget them in the future, as I don't work often on such Multicam projects. Anyway, thanks a lot!