Regarding other elements: there should be a few (probably less than, say, 20) such elements; the problem is, I don't know how I can find them. The edits were done fairly long ago.

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Regarding multiple "wikimedia_commons" keys: https://wiki.openstreetmap.org/wiki/Key:wikimedia_commons says this:

If more images are available, just add the "best" one or create a new Wikimedia Commons gallery

Since "best" is subjective (which is actually mentioned as one of the drawbacks of using the "image" key), I'd prefer linking to a category or a gallery. This also covers the case that a "better" image is added there by a Commons user who is unaware of OSM. If there is more than one category that contains suitable images, the best option is probably to create a new category (if appropriate) or a gallery on Commons, and link to it.

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Keeping both "image" and "wikimedia_commons" keys: sounds to me like a fair solution for the time being.