Well, Cobie Smulders got away from her CBS comedy to film The Avengers and Jason Segal has found time to write and star in movies while still acting on How I Met Your Mother, so I'm sure Dennings could find the time if they needed her.
Not real important though, as I'm sure the movie will do just fine without her.
But Hemsworth mentioned that filming will begin in August, and that's when they usually start filming new episodes for a September premiere. Avengers began filming an April when it's over and Segel shoots his movies during the summer months.
Not necessarily. Scheduling is different for all shows, and the window of time between writing/shooting/completing episodes
and when they are finally broadcast can vary even more so. It is entirely possible that Dennings could still appear in the film with as large as or larger a part as she had in Thor 2. It will all depend on scheduling.