Terry, in reply to your questions about the unsealed sections of road. Living in Sorell, the 800 metre section of gravel road from Midway Point is currently in good condition, flat road, very few potholes, covered in fine gravel, should not be a problem on a road bike. As an alternate route, after having lunch in Sorell, you could double back to Brinktop Road (small hill and only a few k's,) take Brinktop Road all the way into Richmond. Brinktop Road is sealed all the way, and the gravel road section meets Brinktop Road.
As for the road into and out of Piper's Brook Vineyard, maybe there is a local lurking on this forum that might be able to give some details as to the condition of the road.
I have a choice of taking the mountain bike or the road bike on this ride, and am seriously thinking of taking the lighter road bike, and thinking that I should be able to negotiate the few gravel sections.