Hmm, dcraw would, indeed, be the surest and slowest way :-) You make a good point there.

I don't see what you mean about my logic being redundant. Do you mean that exiftool, when overwriting an image in-place, validates its changes before rewriting the image? What about if write errors happen during the final write? Does "overwrite in place" actually not write in place? I say this because I am definitely using "overwrite in place" to try and preserve characteristics.

I'm not concerned about poor utilities. Any EXIF data modification would break those utilities, whether done by exiftool, Adobe Photoshop/Lightroom, Photo Mechanic, etc. I'm expressly looking at modifying IPTC Core information and so on, I expect my image utilities to be capable of handling that :-)