Just for reference , two ways of implementing this function in UI : as buttins [+1][-1] , or as RMB menu

I use option 1 while i paint, that's why i prefer to add another shortcut for this operation.
I like the 3rd option, but i think all the process (A, B,C ) could be in background mode to avoid confusion. Maybe when you paste something with this option, it is already in transformation mode, and when you press Enter it automatically merges with layer below, and user can't even see new animated layer (sounds like a Macros). OR you can choose this in Preferences - to see and control all the process (A, B, C) or not.

At the moment we can't copy and past berween frames (not entire frame but parts of it), it creates new Layer as usual in 'paint mode' .
We can have another shortcut for this operation (ex. Ctrl+Shift+V)