Can you visit the winery without buying a ticket to visit the rest of the estate?

Yes, when you get to the guardhouse turn right. The winery is adjacent to the hotel. A ticket is only needed to turn left and visit the estate and gardens. Enjoy

I don't believe so.

Yes. You can visit the grounds of the mansion without paying as well. You just have to pay to go inside. The winery also gives free tastings, but that might come with staying in a room; I can't remember.