As a seasoned Caravan pilot, this recording sends chills down my spine.
It sounds to me like the pilot encountered moderate to severe icing. The sudden dive could have been due to a tail plane stall caused by ice accumulation. Or the plane could have loaded up with ice and lost sufficient airspeed to enter a wing stall. It's hard to know, which. The blocked pitot tube only added to the pilot's confusion. Pitot heat can only do so much, so I wouldn't jump to the conclusion that the pilot forgot to turn it on.
While second-guessing the pilot is tempting, remember that many Caravan operators provide little if any training for dealing with ice encounters. Even with some training, it can be difficult to tell how much ice is accumulating at night. Most of the aircraft are flown single-pilot, which doesn't help.
My experience is that cycling the boots in the Caravan can have little if any effect. Usually the ice cracks, some flakes off, and the majority stays firmly attached. Even if the ice sheds from the boots, there is the problem of runback to the unprotected surfaces.
The FAA issued an AD that prohibits the continued operation of a Caravan into moderate or greater icing conditions. It also restricts autopilot use. You can read more here:http://freightdogtales.blogspot.com/2006/03/patch-upon-patch_17.html
My three step approach to flying the Caravan in ice is:
1) Avoid - know where you're likely to encounter ice and don't fly there.
2) Evade - never let ATC put you or keep you in icing conditions. Be assertive and declare an emergency if you have to.
3) Escape - when you ice up, have an escape route already figured out so you can shed the ice or land
The Caravan is a great plane, but it becomes one scary ride when loaded up with ice. Kudos to that pilot for regaining control.