P: KBSC for stacking/collapsing ?

Yes. I know that. But if you edit a lot of photos externally, it would be faster to have a shortcut, like the alt+e for external editing that you can assign to a button.

You likely know that, while in Develop, you can select two or more thumbnails in the filmstrip and do right-click > Stacking to access all the stacking commands.  As a workaround while waiting for Adobe to implement this (which could be a very long time), you could likely use the Windows Autohotkey utility or a similar one on Mac to assign shortcuts to right-click > Stacking commands.