1. Heat Wave only has 50 power in rain. While it's good against Scizor and Ferrothorn, that's all it's good at. A super effective 100 will be about as powerful as Thunderbolt, so it really has to be at 200 power to be stronger then T-bolt.
2. Zapdos can survive without a spinner, just so long as you give it roost. I think Subroost looks the best set to use without spin support, but if you want to go offensive, it still does have that base 125 sp.att.
Zapdos is generally better with a spinner, though.
3. Conkeldurr is good, but it takes a surprising amount of damage from bullet punch.
252Atk Choice Band Technician Scizor (+Atk) Bullet Punch vs 120HP/0Def Conkeldurr (Neutral): 44% - 52% (169 - 199 HP). Guaranteed 3HKO. 7% chance to 2HKO.
This is just low enough that you could maybe handle it with Drain Punch, but if your foe swaps to something like Gliscor or Jellicent, it could get difficult.
4. Terrakion is okay...but it can't take a Power Whip, Gyro Ball, or Thunder Wave very well. It's not really a counter, just a check.
5. Conkeldurr is probably the best. Terrakion doesn't handle Ferrothorn as well, and Toxicroak doesn't really like thunder wave as much. Toxicroak is still excellent for this, though.
6. Slow teams are fine, just as long as you prepare for faster threats.
So, how about Choice Scarf Gyarados + Sigilypth? Scarf Gyarados can handle all Volcarona sets besides sometimes the offensive one if it's running HP rock, but even then, it only OHKOs after stealth rock some of the time, and that set may also run HP ground for Heatran. Sigilypth was a difficult one to come up with, but it turns both Ferrothorn and Toxicroak into set up bait, just so long as you aren't fighting the extremely rare Nasty Plot Toxicroak. Scizor also despised being burned, though it's a lot riskier to try and set up on if it's not burned first.
Yeah, it's RU, but I've found that set to be extremely deadly.