Anyone who has seen the construction of a CSU will know that low MP cannot cause a prop overspeed. The only thing that can cause a prop overspeed is a failure of both governors within the CSU.
As I read her post, MLB was between cloud layers and not in icing conditions. She talks about completely different symptoms for each engine failure. It is possible that the second failure was caused by intake icing descending through cloud layers, but I won't bet much on that - MLB was there - she could probably tell you whether it was likely given the conditions at the time.
As for the first engine failure, my money is on some mechanical failure within the CSU.