Back when I applied, I had to book flights - I think that's changed now because it doens't seem to be a requirement on checklists anymore.
Most people I have spoken to say to just put an approximate date when you plan to travel.
In terms of the additional info section, you don't have to put anything there if you can't think of anything. All visa applications (for just about any country) have this section. It's basically for any information that you think that might help or be important to your application that hasn't already been asked or disclosed on the form. It's not a "must fill in" simply a "fill this in if you want to".
Hope this helps.