...since, of course, it can be blocked from view depending on the angle view or other visual obstructions. But it is presumed to always be present on female conopinae and absent on males (except perhaps in anomalous individuals).

In the photo above, it's hard to tell whether a theca is present...as it could be obscured by the somewhat-tightly curled tip of the abdomen there.

Note that female conopines typically have a more ventrally projecting 7th tergite...which "opposes" the theca and is used in conjunction with it to pry open the tergites of a host during insertion of an egg. This is clear in your last photo of this series (and appears to perhaps be the case in the photo above as well).