Quote:...When PASTEing a photo into an "open" photo (setting the inset photo as a layert), is there a way to "unerase" the layered photo when I make a mistake. ..snip...
Use a layer mask, it is non-destructive see:
https://docs.gimp.org/2.10/en/gimp-layer-mask-add.html
https://www.gimp.org/tutorials/Layer_Masks/
Quick example: looks like this:
Quote: In other words, to "fine tune" ERASEing the inset image to match the outline of the base image ?
Look into making a selection then create the mask from the selection. One of the options in Layer -> Mask -> Add Layer Mask.