Is there a way to have multiple ACR versions of the same file, say in this case for different final image resolutions? I think this has been covered before, but my memory fails me.
Yes and no.
As Ted mentioned, the new Snapshots feature in CR 5.2 will let you have multiple versions of the same file. All of the XMP metadata that normally gets associated with an image (e.g., in a sidecar, or embedded in a DNG) gets thrown into a snapshot. So you can have multiple flavors of a single image, e.g., a color rendition, a black-and-white rendition, different crop rectangles, etc. each one as a separate snapshot. (All the snapshots get thrown into the same sidecar file, or embedded in a DNG).
However, the items in the Workflow Options dialog box (e.g., output sharpening) are not per-image settings. Hence they do not get saved with snapshots (or any type of per-image data), so at the moment you cannot have multiple flavors of, say, desired output resolution, image size, and output sharpening settings.