You are correct that He 177s were used over Anzio bay and bridgehead, but KG 40 was based entirely in France and never in Italy (I studied a bit the subject and I am quite confident in what I say...)
The bases of KG 40 in 1944 results to me the following:
At any rate, in relation to your hypothesis of an He 177 landed on an Italian airfield in distress and remained there, no data emerged so far from the very few Italian airfields capable of being used by such plane for an emergency landing. And, considering the thorough work of my friend Nick Beale on the ULTRA documents, the absence of any hint of such an event, coupled with the fact that such a landing would not pass unnoticed (all such bases at the time were shared also by ANR personnel), makes me say that, although theoretically possible, the happening of such fact is in reality highly improbable and it is very difficult to determine an identification of the airfield (let alone its nationality) on such basis.
Thus, (given the He 177 as not "logically" connected to any Italian airfield), to determine an airfield as Italian mainly because an S.81 has been apparently identified is at least debatable, considering that such aircraft was found abandoned also on several German airfields.
Moreover, you cannot certainly exclude the possibility that the "twin fuselage" aircraft was nothing but a captured Fokker G1, several exaples of which were used by the Germans.
No sooner I'll be able to check the VHS I will certainly post further comments, but IMHO the starting assumptions could be wrong and this would unavoidably change the ensuing deductions.