Copying buttons from one layer to another (with accompanying long-press group)

I'm always frustrated when I need to copy a long-press from one layer to another. For example, if I want the punctuation key to behave the same (with the same long-press options) on the default and shift layers, then I have to re-define all those buttons again. I know that I can switch to the CODE page and make the needed hacks there, but it is too easy to get something wrong and blow it up because of a JSON syntax error.

I would like there to be an easy way (within the UI) to copy a button to a different layer.

An alternative would be to make the JSON Code verification a bit more robust, so that it tells me WHERE the problem is (a missing comma, or an incorrect indentation, or something else) rather than just blowing up/crashing the compiler.