Feathering will of course stop windmilling. Unfeathering the prop should mean that at speeds a margin above the stall speed the prop should windmill but that depends on how much internal friction first needs to be overcome. I have no experience of the Lycoming version of the 42 but it is surprising how much speed is required to windmill the prop on the diesel version presumably attributable to higher levels of friction in diesel engines. It is interesting that the Lycoming version appears to require such a high speed to start windmilling. Have you any idea why this should be so?