That report is a good read! Off the top, I find it unlikely that the pilot was allowing the passenger to fly anywhere near those unusual attitudes. Sure, the passenger might fly some more or less straight and level, but not quasi aerobatics. I agree that this type of aircraft can build up speed very quickly when pointed down, particularly when carrying power down. I haven't flown the RV7, but I have flown the RV4, and they are very capable, and clean planes. They are easily capable of pleasing rolling, and getting too far over. A helicopter pilot would probably quite enjoy the solid feel of this plane in unusual attitudes, and let it get too far. Further, a helicopter pilot will be naturally adverse to controlling into higher G. I don't see any reference to the airplane being equipped with an accelerometer. If the G information were not available to the pilot, and his helicopter flying had not refined his "seat of the pants" sensation for G, he might not have applied enough, early enough in the dive, to recover before the speed built up. A spiral dive certainly makes sense in this situation. During flight testing, and intended dives, I've had to apply more than 2.5 G to prevent exceeding Vne (Cessna Grand Caravan). I had temporarily installed a G meter, so I could apply the required G confidently, to prevent exceeding Vne, and it worked. Without the G meter, I expect the G aversion would have seen me exceed Vne. And that was not a spiral dive, simply a spin recovery dive. The spiral would make it that much more difficult, and the moderate power on the way down would have made it all happen really quickly!
The relationship to the US and Canadian accidents appropriately shows that this was not the first time. This is a lesson worth learning! Build up unusual attitude flying skill in low drag planes with great caution!