The thickness would depend on level of skill of controlling flame.
Ideally, the thinner background, the better it is since metal
granules would stiffer metal considerably, but this would depend on
many other factors. If concern is collapsing of the background
during granulation, it can be supported with fire-resisting compound.
Even regular plaster-of-Paris would work. Also, consider using
sterling for the background instead of fine silver. I am familiar
with the literature on granulation and recommendation of using fine
metals. You can obtain fine silver surface by using surface
depletion. In this way you have fine silver to work with and the
backbone of sterling to insure wearability.
As far as druzy, there is no reason to worry. A lot of antique
jewellery was set with glass paste and it still around. Properly
constructed setting is the key to stone longevity. The biggest
stresses stone is subjected to is in the process of cutting and
setting. If it survives these stages, it would do just fine in the
properly constructed ring.