Is the preview function of the layer set not possible?
No, I think it would be implemented when there were machines which are able to render all of this. Each layer-set would be some kind of own PGM-out.
I’d welcome this feature as well. In fact, it doesn’t have to be realtime nor full-resolution.
From my opinion an approx. ~360p preview-snapshot made by opening and refreshing after changing a layers visibility setting just within the heads-up panel would be enough to judge whether or not all needed layers are set correctly.
To make things easy, I’d just show layers set to «Live» and disregard layers set to «No Action».
This would make it easier to make and assess changes during a running show.