Actually if they turn into a spider you get credit the second they turn, but as for the ones that make it the whole way, yes they'll die miserably far before they get there...............unless perhaps there's more than 4 gnomes in the zone, in which case you could keep freeing them until 4 turn to spiders.
Nope, cause there aren't I've opened every single cocoon in zone, only 4 gnomes, unless the script adds another gnome to a cocoon should one die so you dont' have to restart whole mission. /shrug lotsa variables