Blending layers above background layer without background layer interfering

I have solved my problem, but not in an elegant manner so I am asking the community to help educate me. I appreciate any time and effort that is put into responding.

I am using an animation to model a physical system that has several components. The components change over time and some of them interact. I am blending the icons for the components  that interact (using color dodge mode) to illustrate that. I would like these icons to overlay a background that represents the system. I don't want the background to interact with the layers above that I am blending. Unfortunately the background interferes with the icons that do not have a blend mode set to normal no matter what setting I use for the background.

I have a number of animations to make that depend on the state of the sytem and the animations will differ from each other.

I can precompose the layers that contain the icons, which works. but it appears from the user guide that if I use that precomposition in more than one animation any change in one use will propogate to the other animations. I can simply make a new precomp for each animation so this will still work.

I did try to precompose the background layer, but the precomp of that still interfered with the overlying layers.

I can also reduce the opacity of the background. These are black and white sketches and so I simply darken them up in Photoshop and then they are still visible at an opacity of 20%. At that opacity the background does not interfere significantly with the blending.

These solutions seem less elegant than having a way to keep the background layer isolated by setting something in the background layer. It seems like a simple thing and I am surprised that no such setting exists.

I am fortunately able to continue on with my project, but would appeciate some insight into this issue. Thanks