Howdy...it's a bit late for this, but maybe the next guy to read this thread will benefit.
The shaft freewheeling question is one that I put to the Hurth factory boys a couple of years ago, and here is their answer. It does no harm, whatsoever, to the transmission to leave it in neutral while sailin, letting the prop free-wheel. They said that there is a little bit of wear, but it is so negligible that it's a non-issue. On the other hand, if the tranny is not placed in neutral, it very much matters which gear the tranny is left in while sailing. The only way you can stop prop rotation with the transmission is to shift it into reverse. You should NEVER leave the tranny in forward while sailing because water flow over the prop will cause the shaft to exert rotational forces on the forward clutch. This clutch will continuously slip, with it ultimately becoming worn/permanently glazed and therefore prone to slippage under load. The reason for this is that the two clutches (one for forward and one for reverse) each have a spring loaded (or centrifugal) mechanical clamping system that locks the clutch against slipping in the direction that holds the load. The clamps, however, will definitely slip in the other direction and that is what causes all the mischief with wear and tear on the disc.
Hope this helps.
Jim