The PA46 Malibu range nearly all have FIKI ( Flight Into Known Icing ) capabilities. I believe N264DB had de-icing boots on the leading edges of the wings & leading edge of tailplane & also the leading edges of the elevator ( highlighted in yellow boxes )
Attachment 2922
These are hydraulically inflated to break off Ice build on these surfaces ( if they have a pin hole or leak, they fail to work )
It is not known (by me) if they were serviceable on N264DB or whether the pilot understood the procedure to use them correctly
The engine in the PA46 Malibu is a fuel injected engine so carb icing isn't an issue.
My assumptions on the erratic decrease, increase, decrease in altitude are one of two scenarios:
(1) The pitot head froze up & instruments failed to function correctly, thereby the pilot chasing the needles was fighting to keep what he thought was level flight
(2) He got Spatially disorientated & just simply lost control & was pulling & pushing at the controls trying to regain level flight.