I think your way over complicating things. You can access any property on any layer from another layer with an expression. There's no need to store anything.
If you have an animation on a single layer even if it has a whole bunch of effects and different changes then you can save that as an animation presets. Once you have created your own animation preset you can apply that to any layer.
Sometimes it is efficient to take things like tracking data and apply that to a null. I cannot think of a single instance that would make it more efficient to copy keyframes from one layer and paste them to a null just to store them for use in an expression. The only possible reason that you would want to copy keyframes to another layer would be to timeshift that layer so the samethings happen at different times.
As Rick says, the whole idea doesn't really make sense, since you can access keyframes in a million ways regardless to what property they are attached to.
Yes, you can do this. You can also use control effects (Point Control, Color Control, Slider Control, etc.), which are analogous to null layers, to provide controls for expressions. I frequently use these control effects on nulls to provide a central "control panel" of sorts for things like color palettes, numerical settings, etc. that are used on multiple expressions on separate layers and don't necessarily associate logically with any one particular layer. This allows me to label the layer as a control layer and keep it distinct from the visual elements in the composition.
This looks very promising. I will delve more into control effects. I have 12 different outputs from soundkeys wanted to know if there was a better way to organize the keyframes without having several iterations of soundkeys.
I would suggest renaming your various SoundKeys effects to keep them straight, but otherwise you wouldn't need to do anything else. Having a bunch of expression control effects isn't all that different from having a bunch of SoundKeys effects.