Hi, I grew up in Greenwich and saw your post.
Just a few thoughts ... definitely NOT professional high quality comments.
1) I have been enamored by the idea of prefabricated / modular homes. And if you intend to tear down your house, it might be more economical, efficient, and less environmental impact if you "order" a home and have it delivered and put on the property.
2) I've heard that Greenwich has been in a housing slump. I think it has to do with tax changes and in general people are leaving for the Southeast.
3) I think if you have a waterfront property and it can be redeveloped, you can probably find a property developer and sell it to them. These companies attend auctions (tax or foreclosure) to identify opportunities, look for bargains, fix them up nice and then sell them.
My opinion is that if you have a timeline to leave ... I wouldn't dump more money into it. Someone will see the potential by the water.