Is the archer a "foreigner"?

[Anton]

As far as I can tell, William Tell didn't achieve folk-hero status until the very late 15th century or 16th century.

That's not immediately evident. The first known written report is the You are not allowed to view links. Register or Login to view., dated 1474, and since it's a compilation, this in fact suggests Wilhelm Tell (no matter was he a real person or not) having been a relatively widespread heroic  motif already (to be included in the compilation along with the Rutli oath).

Yet I have found no early depictions of him neither.

There are numerous other manuscripts that have a crossbow archer for Sagittarius. Would any of these represent Wilhelm Tell?

Not necessarily, especially those not Swiss, but as I said my idea is that rather than representing a "general" crossbowman, this might be a personification important for the time and place. The Voynich Manuscript imagery manifests, in many ways, its great attention to tiny details, and, as such, certain disinclination to generalization. That's the point.