While you did a good job at identifying the Volcarona weakness, the way you fix it doesn't seem to be the best option imo. I always wonder how people beat Volcarona with their Jellicent. Best-case scenario:
- Volcarona is send out against Ferrothorn.
- Volcarona uses Quiver Dance while Jellicent switches in.
- Volcarona Quiver Dances while Jellicent Taunts (not Taunting would gave Volcarona to set up even more Quiver Dances).
- Volcarona 2HKOes with Bug Buzz while Jellicent fails to do notable damage to a SpDef boosted Volcarona.
- Volcarona sweeps.
A better fix would be as easy as Roar > Will-O-Wisp on Heatran. With Roar, he can take an attack from Volcarona (being immune and 4x resisting its STABs), and Roar him out forcing him to take Stealth Rock damage. Also, this is a smaller change, which is usually preferred. Trying to stick as much to the original team is important.