Are you sure the parenting is the problem? It is possible to sort of turn a parent on and off with expressions, but that gets a bit complicated. Instead of animating your opacity (which could also be the problem!) try just trimming your layers in and out points and leaving the opacity at 100%. So in other words when the parented layer needs to show up trim it’s in point to that frame alt + [ and go back one frame and trim the non-parented layer’s out point alt + ]. Give that a shot!
I’m still surprised the adjustment layer didn’t work… I just want to make sure that was set up right. So basically you should have this:
1 Hand layer
2 Mask layer (parented)
3 Mask layer (unparented)
4 [adjustment layer]
5 face layer
and to just make sure why don’t you change one of the parameters in the curves on the adjustment layer (just brighten it a tiny bit or something) to make sure that AE is actually rendering the layer and not just passing through it.
Hope one of those fixes it! I know how frustrating that can be…
– Spencer