for recruiting an outside expert. Greatly appreciated

Snyder frequently uses the presence/absence/extent of a “prolongation” of the fore metatarsus but that can’t be evaluated in these photos. The only species where Snyder mentions broadened tarsi is Lispe albitarsis which the key describes as having “Three basal fore tarsal segments slender, mostly yellow, the fourth and fifth segments distinctly broadened and infuscated” (couplet 6, page 8). Snyder describes this as a “small” species distinguished by “the shape and coloring of the fore tarsi” and the "very narrow parafacials (in frontal view) which at their narrowest part are not over twice as wide as the greatest aristal diameter.”

I found your photos of this species at Diptera.info (1, 2). The basal fore tarsi on that fly are much more yellow in color and look much thinner than on my fly.

I’ll move these photos to the genus and leave it to you to decide whether the ID can be pushed any further.