Catastrophic accidents like this one rarely have a single cause. The fact that the Piper Malibou made it from Edmonton to near Wainright indicates that overloading, though possibly contributing, wasn't the main cause. Accidents primarily due to overloading usually occur either on take off, landing, or a balked approach or landing. An aircraft improperly loaded, primarily with a centre of gravity too far aft, are most likely to cause control problems, even if the aircraft is under its max takeoff weight.
Without the actual accident report its hard to tell the actual cause of this one. The media focuses on things the general public can understand, such as overloading. What the general public doesn't understand is what happens when instruments start to fail. Even a senior official in the Ontario government years ago complained that pilots wouldn't fly Twin Otters under contract to them that had instrument problems. After complaining that "experienced" pilots should be able to handle these deficiencies a test was done on experienced pilots in a simulator. After given instrument failures all experienced total loss of control in anywhere from 45 seconds to 2 minutes.
The fact that they say the pilot hadn't practiced "partial panel" flying in some time is a bit of a misnomer. It is rarely practiced anyway in modern aircraft, and almost never in the airline world, unless it is because of an electrical fault, and then only until systems are restored, which only takes a couple of minutes.
This situation would have been a handful for an experienced full time commercial pilot, this guy was company president first, and pilot second. Usually not a good match when things go for a sh*t.