[I can't alter the opacity of layers below an Adjustment Layer using that Adjustment Layer can I?]
Yes, if you apply AE's native "Transform" filter to the adjustment layer and keyframe the opacity.
It's not really using an adjustment layer, but similarly you could also remove a portion of an image without directly editing that image layer by using asolid above it masked to the shape you need as a track matte ... or even with stencil/silhouette alpha blending modes.
BTW, it makes me feel better to know I'm not the only one whose brain just flat out stops working at times and I have to go to the forums and ask how to do things I've done a million times. ;~)