Further observation indicates that where two instances of Move Item to Trash appear, the first duplicates, while the second works as intended. Thus, this appears to be simply a mislabeling of the first of these two entries in the pop-up menu that appears upon right-clicking.
Zoter 5.0.58, MacOS 10.13.6

While processing duplicates, simple removal of one of the alternatives may be preferable to merger, but is not allowed within the duplicate search results (only option is deleting both, or merging them)

It's not allowed because merging is a much safer operation. In particular, the items may be in different collections, and they may be linked to from different word processor documents. In both cases, merging takes care of that properly, whereas deleting doesn't.

You can choose the master version from the Duplicate Items view. The only downside then is that child items aren't currently deduplicated in any way (and, often, couldn't be in any automated fashion, because of PDF watermarking), so you may end up with, say, multiple attachments. But those are safer to delete manually.