mushroom is floating (terms correct )
The term is as correct as any other - unless somebody knows what the Murano makers called it. In the exhibition of weights at the recent UK Cambridge Glass Fair, a similar weight was shown (with green over white overlay) and the description was "floating umbrella".
base is polished smooth and has a facet (as I presume all duble overlays have on the base?).
Yes, because the base will not have any overlay, there will always be a clear section, and if this is not finished with polishing, (and perhaps with added cross-cutting or a star-cut), the "cracking off" will show through the weight via the facets.