How about combining the mix and alpha over node as one and calling it a merge node. The merge node can still have the blend mode with some expanded blend options like: over, not, or. Nuke and digital fusion has merge node that does everything that mix and alpha nodes do and then some.

If an image has a alpha channel the merging blend mode would be "Over".

Also in the merge node, give it the option to select which rgba channel/channel layer to process. This way, the multilayer exr passes wouldn't take up so much node screen space in the materials, image & compositor node screen. Let image node have one output and the processing nodes can select and process the rgba, z-depth and other vector data.