I'm not entirely understanding your post, but I can see it is definitely an issue for you. Are you talking about copying to the clipboard, or making a copy/merged layer, or what?
ISTM that just using Ctrl a, followed by Ctrl c will copy a single layer to the clipboard, and Shift Ctrl c all layers. Is that not working for you?
Shift Ctrl Alt e makes a copy merged layer immediately above the layer selected when invoking the shortcut (providing that layer is switched on.
If I am misunderstanding you, can you give a bit more information?
I don't get why you have to use the shift key, since it is a single layer, it already is merged, all that is required is copying the layer. Or better yet, use the alt key and drag the layer or document depending on what you want to do with it.
When it comes to multiple layers I have to agree with Trevor, ctrl-alt-shift-e is my go to command for making a composite of all the layers.
Yeah, I'm talking about copying to the clipboard. I'm often doing projects that involve copying various image and placing them into others. The fastest way to do it (when both files are open at the same time) is to do select all, then SHIFT + CTRL + C then paste it into the file I want.
However, not all images I'm copying from are multiple layers. Some are single layer. It just bothers me that I have to look and see if the file is a single layer or not and then remembering which shortcut to use. It's not a huge deal or time waster, but it's just annoying. I can't think of any logical reason as to why SHIFT + CTRL + C doesn't copy all layers, even when it's just a single layer. It's not like it's illegal for a shortcut to overlap in function with another one...