Be Happy in your Game!
I spent 100 Geek Gold and all I got was this lousy overtext message!
Found it. Thanks.
5.1 German Armies under the control of the German Allied faction
German armies which start any round in Fronts 5-6 or 9-11, are
controlled for all purposes by the German Allied faction, and
operate during the German Allied rounds (only).
It does say that the units must start the round in that front, so you are correct in that you couldn't move them in the German turn then use them as a combined force or lead unit in the AH turn of the same round.