Your observations are correct... once the "Cntrl+Alt+Shift+E" operation is complete, modifying anything below the merged layer has no direct effect on the merged layer or any layers above it.
re:
Is there a way to make a layer that adjust all layers below, leaves layers below also editable, and is not "transparent"?
No, but it would be a neat if it worked that way...
One approach is to apply modifications/corrections/edits to individual layers first, then add the adjustment layers -- and avoid doing the interim "Cntrl+Alt+Shift+E" layers. That way you can fine tune any of the individual layers or adjustment layers after the fact w/o having to rework interim merged layers.
re: Whitening teeth trick (Scott Kelby method)
* Press Q and use
Quick Mask to "paint" a selection for the teeth. When selection is painted press Q again to exit
Quick Mask.
* With the selection active create a Hue/Sat adjustment layer. (Note black mask with white where teeth are located. The mask 'isolates' the effect you're about to apply.)
* From dropdown menu, choose 'Yellow'
* Move Saturation control left to remove most, but not all of yellow
* Move Lightness control right a touch to brighten
* Click OK when you're happy with the result.
* Zoom out... fine tune Opacity if needed.
Easy to adjust later, too.