I guess it is possible for inertia to be enough to rotate the cam past the valve spring without any load assistance from the crank but I think this is unlikely given the strength of the springs. The rpm factor of the crank though does make sense when I think about it some more. The sheer rpm of the crank which would be faster than the inertia powered return speed of the cams would cause the collision, compounded by the cams spinning at a slower rate one of the pistons is likely to hit TDC before the valves get out of the way. Still think it is worth peeps getting the tests above before writing it off. You could get lucky and if you do buy a lottery ticket!