Those pics were not so helpful, but here is my advice: when I've had cupped trailing edges that were bad enough to bother me, I've just CAed a piece of 1/16" balsa (or whatever thickness is necessary) onto the concave side, then block-sanded down both sides to get the straight edge I wanted. It helps to draw a line with black marker for the edge you want, then sand down to it. Try to match the same thickness as the other elevator. Two mm is not awful bad, but depending on how picky you are, it's not hard to fix.

This generates a lot of balsa dust, which I save in a plastic bag to use for filler. Just mix the balsa dust with some aliphatic glue into a putty, knife it on, let it dry, and you get a pretty strong gap filler. Sands easily enough. Don't use a lot of glue--just enough to make the dust stick together.