^Yes, the Whedon fanbase isn't huge, but the Marvel Cinematic Universe fanbase is. And I doubt very many of the MCU fans are going to pass up watching a TV series created by the director of The Avengers and starring Agent Coulson because they'd rather watch NCIS. So even if you leave out the Whedon fanbase, the show still has a solid built-in audience.
Maybe, but success at the movies doesn't have to translate into a successful tv show, it certainly has a better chance than other shows but it's not guaranteed.
In the past there have been quite a few hyped shows that crashed and burned in the first season, remember Studio 60 on the Sunset Strip? Aaron Sorkin, great cast, lots of publicity, what could go wrong? We all know how it ended.