Working with layers in the Animation Palette is super frustrating. I don't know why they hobbled it by making it impossible to copy keyframes from layer to layer, or even from one element to another within the same layer. All you can do is make new keyframes and option-drag to duplicate them. Also, the inability to collapse a single layer doesn't make sense. There is a workaround if you don't mind deleting all layers but the one you want to collapse, collapse it, copy the resulting merged keyframes and then paste them back into the base layer of the original.

I suspect problems with layers in Add mode are caused by conflicts with the Min Limit and Max Limit values in Parameter Settings. As mentioned in a comment above, the settings affect existing animation. You can fix spline overshoot by changing the values, sort of like compressing or limiting audio. For example, here's an animation with keyframes set at -5 and +5, with spline overshoot caused by moving the two middle keyframes closer together.

Changing the Min Limit to -5 and Max Limit to 5 with "Force Limits" selected flattens any value exceeding the limit and adds a representative black line, as shown below. Any keyframe with a value between 5 and -5 is unaffected. This is non-destructive, so you can always go back.

Here's the same animation with the Min and Max Limits changed to 2 and -2. This time all the keyframe values get changed and the overshoot is flattened as well. The flattening can still be removed by increasing the Min and Max Limit, but you can't Undo the change to the keyframes.

Here's another animation with keyframes set at -9, +9, -7, +7, -5, +5, -3, +3, -1 and +1.

And here's what it looks like after changing the Min and Max Limits to +4 and -4. Any keyframe values exceeding the new limits are changed. The rest are unaffected.

The graph can be confusing when working with layers, partly because the display doesn't update when you select or deselect "Include in playback" or change Composite Method (Add and Replace). To refresh the display, select a different layer and then reselect the one you're working on. The red line represents the currently selected layer. The black line represents a composite of all the layers that have "Include in playback" checked. Unlike with the base layer, changing Min and Max Limits doesn't change keyframe values, it just flattens non-destructively the same way spline overshoot is flattened.