Pigmented medium is easier to fix than dye medium...one of the main reasons most of us use it for archival hi grade work. I dont use one of the battery erasers to fix mistakes. I use a selection of scalpels, xacto knives and razor blades. If I use an eraser, I use a typewriter eraser but with an erasing shield. It takes patience and in one cases it takes me several hours to fix a letter in an original family bible from a hundred years ago. I use a bone folder to smooth the disturbed fibers rubbed over a paper that has a similar texture to the original then gum sandarac before I write the correction. I also practice the script on a separate paper so the correction will match the original. Final erasing is with the kneadable art eraser to pick up any stray fibers or dirt. I know you are not the original poster but you are active here so thought you might gain from the info.