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Hi Claudia,
I like your comparison and ideas.

The main obstacle I see right now is to get enough specimens of X. stackelbergi to check different features.

I am actually astonished about the reduced extension of the wing spot in your lower fly. That worries me a bit about the reliability of that feature.

The clear triangle I am actually not sure about. First, with the overall lighter wing spot in your X. stackelbergi, the contrast to the triangle is of course much weaker, but I still can make out the shape in your photo. Also, in Steven Falks (dead) specimens, although the wings are not straight, one can see lighter areas in the cell as well.

Body shape also is a critical feature, as the segments are quite flexible. An overall comparison between many individual would be ideal, and maybe you are right in that the borders of the third segment are more straight in X. dives, but I feel not confident enough to apply this feature right now.
For example, see here: https://www.inatu...s/41689370
These photos all should depict the same individual but the abdominal shape is quite different.

I hope one time I will be able to catch a male stackelbergi, to have a closer look by myself.

Thus far, it is still an unsatifsyingly described pair of species, but that makes it interesting at same time :-)

Cheers!
Lorin