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I don't know how D. mercatorum is supposed to look like.
According to Bächli's key, there shouldn't be a dark costal spot as in D. repleta - but the size of the light lateral spots and the thin distal stripes on the tergites (>1/2 width in D. repleta vs. ~1/3 in D. mercatorum) raised my suspicion here.

Additionally, Jan Maca once mentioned in a comment that the presence of the costal spot should not be overrated.

So, here's my fly to be discussed.

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Can it be D. hydei?

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No, D. hydei is much darker, with no lateral light spots.

Here's one from today:

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Ok, that's how my D. hydei look - I thought it were reflections.

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Difficult to judge. The pale lateral spots on abdomen look strange, as they reach anterior margin of tergites. Also, the tibial bases have a slight darker band and the gena seems wide - these are characters of D. buzzatii. It would be interesting what a molecular analysis would say.

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I didn't consider that species, as there is no Opuntia around - but that species might have adapted to other food sources meanwhile?

Exciting! Unfortunately, it got startled by my attempts to get better pictures...