So after a quick search on the flak sight, I stumbled upon
this page (I recommend to read the whole article, IMO it is pretty well written and straightforward).
The relevant part regarding the dual-role usage is this:
"
After the experiences in Spain, proper direct fire optical sights and dedicated armour piercing ammunition were developed. The Zielfernrohr 20 telescopic gun sight wasn’t any old gun sight or range finder: it was amongst the best optical gun sight systems in the world at that time, and remained so for most of WWII.(5)"
With the citation being "
(5) For AA work the Flakzielfernrohr 20 telescopic site was rarely used. Most often AA fire was aimed using the Ubertragung 30 predictor firing system. "
So this gives some answers to my question about the empirics of the dual usage. But I still didn't learn anything about how the Flakzielfernrohr 20 was used against aircraft when it was neccessary
in practice.
But I agree with Surfbird that the ingame flak 18 needs refinement. Maybe positioning the gun sight that it won't get obstructed at the most common elevation enemy planes flying by would help as a gameplay enhancing abstraction. Also it would boost german bias.
Still waiting for an expert's input, though.
Another
source. regarding the Ubertragung 30 just to be complete:
"
This predictor system was an electro-mechanical device" [used at a gun battery level, with] "
each gun platform [having] a series of electrical connections that allowed data transmitted from the battery commander's predictor (data transmitter) to reach each gun's predictor system (receiver)."
Note that only the FlaK 37 used an upgraded predictor system, the "
Ubertragung 37", which was "
[...] two sets of dials, one operated electrically from the data transmitted by the commander's predictor, the other mechanically linked to the gun. The crew simply adjusted the controls until the gun's pointers matched those of the predictor"
Also keep in mind, that even though most common descriptions describe the FlaK 37 used purely only for AA, there's at least one instance of an example equipped with the Zielfernrohr 20 to be used against ground targets. (last page of Jentz's book)