I don't think you need a clipping mask. In fact, I think the clipping mask is your problem entirely.
This simply requires a standard layer mask which is unlinked to the layer, so the mask moves without the layer contents moving.
Lock transparent Pixels means.... don't allow transparency alteration to any pixel which has transparency.
Why you appear to think you should be able to then alter the transparency is a mystery to me. The lock transparency command could not be any more clear in my opinion. This command is helpful when you have partially transparent pixels and need to alter ...
Your selected area is, in fact, empty.
It doesn't contain transparent pixels it just contains nothing.
You can select it because you are not selecting 'pixels' as such but simply 'an area'.
Filling the area with the paint bucket etc it then defines that area to become coloured & then will contain pixels.
Trying to move the selection with the Move tool is ...