It's probably better to leave the last modified date alone (actually restore it) when updating metadata only.
I agree. A purist would argue that the file has, in fact, been modified, but in this context touching the mod date is sort of overkill. If the metadata are displayed in the album, the change will still get picked up, without reprocessing the image.
ETA: But I'm sure there's some situation we're overlooking. I just can't think of what it might be, at the moment.
I now have b11, but adding a title to an image still causes it to be reprocessed. And I am not writing any metadata to the generated images.
ETA: An observation: it appears that adding a title to an image that didn't have one before triggers reprocessing, but changing an existing title does not.
ETA: And I can almost "feel" why that's the case. An image that doesn't have an associated .info file at all suddenly gets one when you add a title to the image.
I suspect you're right. The question is how to handle this...