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Now with the nice dissections of the male genitalia, things become clearer! This is Leucopis palumbi Rondani, 1872.

Raspi (1983 - Frustula Entomologica Nuova Serie 6 (19)) clarified the identity of this species, studying the types. Tanasijtshuk (1986) described this as a new species - his Leucopis ulmicola. The genitalic illustrations for both Raspi and Tanasijtshuk are identical to your images, and the descriptions and specimens I have match the external morphology perfectly too. And the puparium is rather unusual for Leucopis, but is just as described for this species.

This species is mainly known to attack galling "aphids" (pemphigids, eriosomatids, fordines) on Pistacia, Ulmus, Populus. But never Fraxinus up to this point - were these associated with galls? This is the first time known to be associated with aphids on Fraxinus, but a wider host range is not unexpected.