Ok I think I understand your request, even though there remain a few points that bother me in your logic 
In the end we get a bit lost because in this thread you express 2 distinct requests.
Your request No.1 :
- Create a virtual device that groups the features of several physical devices, so that there is only one left to test within your scene. (maybe this ties into the notion of a « super device » I had already mentioned here > Créer un super device virtuel (which was a brain fail for me ^^)
Request No.2 :
- Change the way a scene works > the famous toggle ONE <<>> ALL which cannot make sense insofar as, even if I understand your logic, Gladys must process information one after the other to move forward logically. In other words, a sensor’s state cannot be a trigger. Only the change of state of it can be a triggering event.
So your logic which is:
for the scene to execute, meaning for the alarm (total in this case) to activate, it requires « user absent » AND « all total-alarm sensors OK »
We can turn it around any which way, it cannot take this form in Gladys (nor elsewhere I think).