I get what you’re doing with the multiple layers, but don’t quite understand what role the null and camera are playing. Why is the camera rotating?
From your description, it sounds like you would want a null object in the center of each letter. Parent all layers for the letter to it’s respective null. Then animate the Y rotation of each null.
I’ll add that the position expression on each one of your letter duplicates should be based on the position of the front one. Like this:
masterPos = thisComp.layer(“first letter”).transform.position;
masterIndex = thisComp.layer(“first letter”).index;
spacer = index-masterIndex;
[masterPos[0],masterPos[1],masterPos[2]+spacer];
This assumes your duplicate is below your first version of the letter. Then you can simply duplicate it a bunch of times, then parent them all to a letter null for the rotation animation.