In a little simple way. Duplicate the desired group. Merge it. Hide the original one. Create the necessary adj. layer above merged group layer and then merge both layers, right? But then using smart object is better since it keep your groups alive and you don’t need to go through hiding, merging and such things.
Not exactly, because applying images on the front (not on the back smart objects) will multiply their blending modes. Thus, the effect may vary when working on another ‘sheet’ – i.e. modifying stuff from/in the smart objects.All in all, both methods are quite similar, but you’re water-proof when working right on the upper-most area, which will always be your layers above the original layer.
Yep not exactly. But still if you do not want any base effect over your current image, I doubt we can go for apply image. Thus, it depends on user to user.
Scripting is the solution for most of these silly things.