What I would do is devise whatever "plan B" you can for the additional night at another hotel. Then when you check in at Napa tell them you'd like to extend your stay by an additional night if something opens up. Based on my 25+ years of experience, there's a good chance that something will (but that's never a sure thing).

I'm personally not a fan of the Napa Marriott. I'd actually choose the FI in American Canyon over the FS property. In actuality, Mrs. Cyberdad and I like another non-Marriott branded hotel in Yountville, and usually stay there.